Playing it by ear? reccomend or not?

If you’re not picky on lodging and have a flexible budget, you’ll find a hotel easily in any of those cities.

It’s not that you won’t find a hotel without a reservation, but expect to pay more if you don’t book ahead. You may not get to sleep in the Bari Gotic either. If the price gets too high you could find yourself staying in a not too nice area.
Book a reservation for Sagrada Familia before leaving home unless you want to stand in a very long line.
When you’re ready to move on, check: https://www.renfe.com/es/en or go to the train station to buy your train ticket. Chances are you may have to transfer instead of taking direct trains.
Yes, four nights is enough for Barcelona.

By "3 days in Barcelona" do you mean 3 nights or 4? If you're flying into Barcelona and staying only 3 nights, that is definitely not 3 days; it's really only 2 full, non-jetlagged days. I would definitely not recommend such a short stay (but read on).

How much time you need in any city depends on what you want to see and do while there. I love Barcelona and especially like its modernista architecture. I also enjoyed the several art museums I visited. I spent nine days in the city and went back for another, quicker, visit three years later. YMMV.

In addition to La Sagrada Familia, you will probably need to buy entry tickets ahead of time for the following additional places in Barcelona if you want to see them without spending an annoying amount of time in the ticket line--and perhaps being told that all the day's tickets have been sold if you show up in the afternoon.

Casa Mila/La Pedrera
Casa Batllo
Parc Guell
Picasso Museum
Palau de la Musica Catalana (English tours do not run constantly)

For your stop in Granada, you need to get your Alhambra ticket immediately. Check now to be sure the 14-euros tickets aren't sold out. If they are, come back here so we can try to help you. I don't think it's safe to wait on this. The Alhambra frequently sells out completely. As I write this the only days in October for which the 14-euro daytime tickets are still available are tomorrow and the 26th through the 28th. The website indicates few tickets remain for October 27 and 28. If you play the Alhambra by ear, there's quite a good chance you'll have no opportunity to see the best part of it (the Nasrid Palaces) at all. At the time of my April 2019 visit, even the nighttime tickets were sold out for at least some days.

In Seville there can be a very long line for the Alcazar. Buy that ticket before you arrive in Seville. (I've never heard that the purchase needs to be made very far in advance, unlike Alhambra tickets.) The line for the Seville cathedral can also be very long. Rick recommends buying a combo ticket at the Iglesia del Salvador, where there should be essentially no line. As far as I know that tip still works, but I used it in 2019, and ticket procedures could have changed.

We just returned from a similar trip to Spain but were there for 4 weeks. As mentioned, if you are not picky or cost constrained, accommodations can be found. They probably will not be in central locations and will require transport to the sights and action. The buses are cheap and frequent everywhere. Vueling flights to Granada purchased less than 2 weeks out cost about $25 more and you may not like the schedule. Our 1025 flight was packed full. (The Grenada airport bus has various stops. We used the Cathedral stop for a short walk to our apt at Puerta Nueva). The Granada to Seville train goes to Cordoba first but with right train no transfer. We took the bus - direct, cheap, comfortable and about same time. Train from Seville or Cordoba goes to Madrid-Atocha then transfer to Toledo train. Our Cordoba-Madrid train was full as was the Toledo day trip train. Bus for Madrid -Toledo is alternative.

As for stay times, it depends on what you want to do. Even now, you may not get favorable entry times for Sagrada Familia, Casas Mila or Batllo, Granada Alhambra, Cordoba Mesquita, Seville Cathedral, or Madrid Prado. Option would be to sign up on the spot for a tour guide. These attractions were packed and ticket lines were LONG. There were a lot of tourists everywhere from all over, including a lot of Spaniards (they just had limitations lifted). Masks required for transportation, some stores and some attractions.

You've got some good info already. Barcelona is a great European city worthy of more than 3 days' visit.

Madrid-Granada took us about 3.5 hrs. by bus. I think the train route is up and going since we were there. Alhambra is a must see, and it's a tough few hours climbing steps, etc. Granada also has its gypsy caves with their music shows and flamingo dancing.

With a few selected websites, there's no reason you cannot make room reservations on the fly. We carry a tablet with us as they're easier to deal with making reservations than a smartphone.

We flew from Granada to Barcelona in an hour on Vueling--cheap.

Been very busy these days before my trip.
But I want thank you all for the outstanding info and suggestions.

so far I have hotels in Barcelona, Granada, Cordoba (but not Sevilla or Madrid). plan is 3-4 nights in each place, how many nights would do Madrid 4? or 5?
I've talked to someone that didn't care for Madrid??? And told me 3 !!!

(also I have No trains trips made from granada, cordoba, Seville to Madrid) will probably play by it by ear, errrrrrrrrr
I also booked some tours in Andalusia in advanced all ready.

is the city tour hop-on-off bus worth it in Barcelona and Sevilla???

How long you stay in Madrid depends on what you want to see and if you wanted to take any day trips. If you don’t care about museums and don’t take any day trips, then 3 days should be sufficient. You are correct, some people don’t like the city while others do. It’s all personal preference. The same could be said about any place tourists visit.

In Madrid, try to visit the Royal Palace, Prado Museum (plan on spending 4h or a half day there) and the Centro de Arte Reina Sofia. If you’re there on a Sat night get out and enjoy the evening paseo (stroll) with the locals and other tourists. I couldn’t get over how loud Madrid was, I’ve never experienced anything like it.
A great day trip is Toledo that’s a 45-minute direct train ride away. When you arrive follow the crowd and hop on the bus to Plaza Zocodover where most people get off. I suggest sleeping in Madrid four nights but if you stay three nights skip Toledo.

As to “play it by ear”, that’s how we travel Europe. We pick a country, read Rick’s book, and block out dates. Once we book the round trip flights, I book a hotel in our destination city for a few nights after arrival and a hotel next to the airport (usually corporate) for the night before our return flight. Those are the only rooms I book before we cross the ocean. We sit down with a calendar, a map, and the book and pencil an itinerary. We make changes as we go. We’ve crossed out towns and extended others. Of course, we never travel in the summer. In autumn and spring, lines are short and rooms are available.

Things sometimes surprise. We’re in Spain right now, and we packed for colder weather than we’ve found. Playing by ear requires being flexible and ready to shop.

I thought about "playing it by ear" when we decided in Sept to book a 14 night trip to Spain in November. In the end, we booked all our hotels for BCN, VAL and MAD and trains in between. The hotels can be cancelled for free and the train tix were so cheap, we can still change plans on the fly and not suffer any huge loss. However, I have noticed that the prices for our bookings have jumped and some availability has decreased as we get closer to the trip. In your case if you are only a few weeks out, there will be no big difference.

One thing is that is still "fly by our pants" is that we have not booked one attraction yet. We are in low season and have 4 - 5 nights in each city which gives us flexibility. I also see that I can still book all major attractions today for within a few days from now, so I am not too worried about availability in November.

Play it by ear, NOT.

I did this on weekends when we lived in Germany, until we visited a town and all the lodging within 50 miles was fully booked due to a festival.

ALWAYS book your lodging and plan your trip, you will save time and money.

Three days is good, recommend Barcelona Day Tours. Add more time for a day trip to Monserrat.