Agriturismo and Tuscany drive or relax in Assisi?

Edit: I am assuming this is the 'tour' you are meaning? https://www.ricksteves.com/europe/italy/itinerary
If not, please direct us to the link, because an organized tour of Italy for 3 weeks, I could not see.
May I ask what the purpose of your travel is? Is to have a brief taste of a lot of Italy or take the opportunity to get to know a little of a few of these places. People who travel independently tend to state that every town move loses 1/2 a day. So even removing Dolomites, you may be losing 5 of these 20 days. I would never undertake this trip independently. I am afraid that Rick does a disservice when spelling out these itineraries, because he builds them with decades of experience and people on the ground to help him - you don't have these.
However, some people like the 'tapas' (taster) method of travel and in the end knowing they loved trevi fountain, but can't remember if they saw that in Rome, Venice or Orvieto. If that is still your taste, I would schedule that day to sit down, drink copious glasses of wine and nap, or forego the wine and just sleep all day. Even on an organized tour, this would be exhausting.

Is the itinerary really busy?

I looked at the itinerary and for me it is very busy. You say you are following the tour. How will you travel from place to place? If you want to essentially duplicate the tour, why not just take the tour? It would sure save a lot of logistics. If you aren't going to take the tour, use it as a general guide and customize it according to your interests. Personally, I'd do about half the number of stops in the allotted time.

Maria:
Thanks for your reply. We are not following a tour. We are following Rick Steve's itinerary. We are doing the booking ourselves. The itinerary is in that link that you posted. The purpose of our travel to really to explore and see Italy.

DougMac:
Thanks for your reply. We are exploring Italy by ourselves, not following a tour. We are only using Rick Steve's itinerary like I said. Most of our traveling is by train, and the hill towns by car.

We stayed at an agriturismo for six nights and it was well worth it for that many nights. It was outside San Gimignano so we did day trips to Lucca and Volterra while also exploring San Gimignano and the surrounding area by car. What was so nice is we had a two bedroom apartment we shared with friends. We could take our time and enjoy the surroundings. We would explore during the day, pick up some food in the markets and enjoy it in the evening back at our place while watching the sun set with the town of San Gimignano in the background. The owner was informative and kind. The grounds were lovely. Fellow travelers were friendly and shared their adventures and tips over breakfast. It was my favorite part of a month trip in Italy. I don’t think you would get that experience with a one night stay.

I haven't gone all the places on this itinerary but my two cents is that there is little consideration of the travel time between places.
Personally, I would cut out some more destinations. I loved the Amalfi coast but it is much more difficult to travel there than other places. Maybe cut those days out and add to other destinations.

We have good friends who did a 2 week tour on their own of Italy. I thought the pace was crazy but they were determined. They had a wonderful time but near the end of their trip when they got all the way to the area Pompeii is near they were too exhausted to even visit.

You have more time but it is even harder to keep up a brutal pace for three weeks.

One and even two night stays are exhausting. We did a 10 night trip to Amalfi coast including Pompeii and Naples a couple years ago. We stayed 2 nights in each place and were ready to come home at the end because moving around so much wore us out. We learned our lesson. We went last year to Greece and stayed 3-4 nights in 4 places (14 nights this time) and could have stayed longer.

Do you have to schedule something every day? When will you do laundry, or just rest?
I like Rick, but I think he skirts some transport time issues, such as going from Venice to Cinque Terre and then just spending a night there (when do you hike then?). My advice would be to add any extra time to places you are already visiting.

What others are saying is you have too few nights in each place. Every time you change locations you loose at least a half a day. The pace that our host uses is much too fast for first time travelers who are unfamiliar with the logistic of transportation.

For a first trip, I personally after 15 trips to Italy, would do no less than three nites in each location. Expanding time in each to do short day trips. Skip the Cinque Terre and the Dolomites and Assisi. You need to look up train times for all transfers between cities.

Arrive Milan, train to Lake Como (Varenna)
3 nights Varenna.

Varenna-Venice. Train.
3 nights Venice.

Venice-Florence. Train.
5 nites Florence. Easy day trips to Lucca/ Pisa (combined) Siena, longer to Cinque Terre ( about 2 1/2 hours each way.) but doable with an early start.

Florence-Amalfi Coast. Train 4 nights.

Amalfi Coast-Rome. Train. 5 nights.
Day trip to Orvieto is easily done from Rome.

To look up train schedules use Trenitalia.com.

Edited to add. And fix%*# autocorrect

gerri has done a great job of laying out an itinerary that will allow you to experience and see a lot without becoming exhausted. Rick’s fast-paced itineraries do not allow for the self-guided among us who have to take time to find our way whereas his guided tours have an expert on hand to ensure no one gets lost. You will need time to orient yourselves everywhere you stop to lay your heads, only then can you get out and see. A two night stay followed by another two night stay followed by yet another two night stay becomes a blur.

Editing to add, I think every trip needs a “centerpiece,” a location where you settle in for longer than any other stop so you can sleep a little longer, establish some routines (go to “your” bar for coffee every morning), catch up on travel planning, communications, laundry. In a 3 week trip you will need a “day off” from sightseeing to do these things. In that location, consider getting an apartment.

I also think Gerri's suggested itinerary is much more enjoyable than adding more sites.

If you do want to spend a day exploring the hilltowns, take a night from Rome and add it to Florence. That gives you 1 1/2 days in Florence and 4 day trips.