Italy in late May or early October

I'm stressing about trying to plan everything with only 3 months left.

You got enough time to plan. High prices nothing you can do about that except go in November and right before Xmas those our extremely popular places it seems

Is the weather in late September/first week of October still pretty decent in Venice, Lake Como, and Rome?

Yes.

Seems like plenty of time to plan for me, too. No need to overthink think things. Venice, Lake Como, and Rome shouldn't be hard to do.

Have you looked at Booking.com? Use the map feature and filter features - set a maximum price. You don't need the perfect place if you are on a budget. Venice is amazing enough that having a modest hotel shouldn't spoil it. Last May, I booked a place at the last minute on Booking.com for one night in Venice for about 90 Euros/night - a place that wasn't available at first. People cancel, availability changes.

I'd go on Booking.com right now and book tentative places in each city that have free cancellation (read the terms carefully! it varies by property. Cheapest prices tend to be non-refundable). Then continue with your planning and check your hotels for more options every few days as you have time. Maybe better places/prices will come up.

I've had good weather in both May and early October in Venice - so who knows? I'd go back again at either time of the year.

I don't know if you are looking at hotels or what, but you may get those prices down by looking at Airbnb.

In just a couple of minutes searching I found this great, beautiful apartment in Venice for $152 a night for May 21st.
https://www.airbnb.com/rooms/2873890?location=Venice%2C%20Italy&adults=2&check_in=2018-05-21&check_out=2018-05-23&s=Ryey-0gK

Take a look. I hope this helps. And by the way I have absolutely NO connection to AirBnb whatsoever

In a very fast check of "Cheap Hotels in Venice (google) I found this:

http://www.cadeldose.com/

Secondly if you are using "booking sites or aggregators" such as Booking.com or Kayak try going directly to the hotel. These sites will tell you all the rooms are gone. But that is ONLY for those sites, NOT necessarily the hotel. As I am sure you know, how this works is that individual hotels allocate a certain number of rooms to these sites (which can be pulled at any time by the hotel) So while the booking site tells you AH! No More rooms Here! check the hotel directly. Also many times the price is cheaper by booking directly. Hotels pay commissions to the booking sites and use them to fill holes in their occupancy numbers.

Hang in there you will find something. Just keep looking. Surely you can find something under $200 if I can in a minute or two.

Good Luck

We went the 1st 3 weeks in October and had perfect weather - in 21 days there, we had one day of rain and it was in the low-mid 70s. We did Venice, Florence, Rome, Sorrento.

Andrew, I have been using Booking.com to read reviews, get an idea of prices. I usually try to book directly through the hotel though on their own website.

So far I've put a hold on a hotel in Rome (Centro Cavour in Monti) - I want near the Pantheon/Navona, but that area is too expensive, and I just put one on Venice, but I'm not crazy about it so I'm probably going to cancel. Venice is the hardest for me to find right now. They're already 99% sold-out on booking.com, and everything decent is $300-400 a night, and I need 2 rooms so that's not an option. Considering dropping Venice to 1 night instead of 2 due to cost.

I'll keeping looking and see if anything else opens up.

Which night(s) in Venice are showing 99% sold out on booking.com ?

Why not try Air BnB if you need two rooms? That apartment suggested is a 3-bedroom and sleeps 5 people. The price actually comes to $179 a night with fees, but that is still very good for Venice.

Venice is always somewhat expensive. I'm a budget traveler myself, so I understand the feeling when you see prices that are so much higher than what you were hoping to find. My response is to settle for really tiny places, but that is a lot more workable for a solo traveler like me.

I would not cut the Venice stay to one night, because at that point you have so little time in Venice that you won't get to see very much. At some point it's OK to say, "Not this year" and spend the time somewhere else. I hope it doesn't come to that, but it's not like the only worthwhile places to see in Italy are Venice, Lake Como and Rome.

Yes, the weather will be excellent then. Go to Austria first saving Italy for possibly some cooler weather.

Before you make a decision, look at hotel rates for fall. It's possible that the best deals for May are gone while there are still bargains for later in the year. Or you may find that the prices you're seeing are the same prices later. October is a lovely month in Italy. I don't think the weather's much different in late May than in early or even late October. . . . mostly warm and sunny, not usually hot like summer.

You can also stay outside of the historical center if you can’t find anything reasonable. I checked the hotel Tritone in Mestre, in front at the station, for the end of May, and they charge only €150+/night, and that is an upscale modern hotel across the Venezia Mestre station (less than 10 min from Venezia Santa Lucia station).