Italy itinerary (help please)

Most new travelers to Italy that do not have a specific regional interest usually go from Rome to Florence to Venice in that span and may include a day trip to other places from those 3 base cities. You can accomplish that easily by train or automobile but I would only recommend an automobile if you plan on going somewhere not served by public transportation as many here do not know that the rules of the road are different and failed to do the research and end up getting ticket sent after the vacation. For the new traveler It is best to sit down with a guide book electronic or paper to get an idea on what you are interested in seeing instead have random suggestions. It is more organized that way, Nowadays you can sample or take out for free many different guidebooks in electronic or paper format at local public libraries. Think of guide books link reading assembly instructions.

In addition to the suggestion to peruse a guide book, you may want to look at the RS Tours in Italy and look at the "Itinerary' section of each to get an idea of what is available and determine what interests you

https://www.ricksteves.com/tours/italy

What would you like to see? Are you interested in art, architecture, hiking, old vs. new etc.
Are there places that you have heard of that you want to go? Do. you want to experience Venice, see the Coliseum etc...
With a 3 year old you will be somewhat constrained...but you can certainly have a great time.
Keep in mind that things are pretty busy in September/October

Thanks all for the comments.

My plan is to see main sights, avoiding bus travel and prefer train or car.

4 +/- cities

How busy are end sept.?

Is Spain the same time better option?

Regards

It will be busy in both Italy and Spain. Go to Italy and visit Venice, Florence, Rome in that order.
Fly into Venice, out of Rome and take trains between cities. You can arrange for day trips from the cities if you wish.