Parma - Modena- Bologna

Parma is closer but Bologna has faster trains, so both can be reached in one hour.
If you start early and move quickly you might even be able to visit the main sights of both.

Parma is a beautiful town, with a gorgeous lush park. It was called the “little Paris” because it was ruled by the duchess Maria Luigia, Napoleon’ second wife, after Napoleon’s demise. Parma is the closest one to Milan, it still will take more than 1 hour by car, about the same by train, but actually I’d take the train: between Milano and Bologna (and back) it stops in every city you mentioned and there are many trains during the day (maybe less on Saturdays and Sundays, you have to go and check on the Trenitalia website).
If you want a glimpse of Modena you can watch Aziz Ansari’s “Master of None”, on Netflix I think, the very first episode of the second season. Aziz Ansari stayed in Modena for real for a few months, he learned how to make pasta and worked in a fresh pasta shop. If you are in Modena in the morning you can visit the Mercato Albinelli, a very colourful market where you can buy fresh produce, bread, cakes, cookies. Bologna is beautiful, bigger than the other two, more chaotic, a bigger city centre, and in general has more things to see.

Bologna was OK, but we loved Ravenna.

Bologna is definitely a larger city, but has a great historic district with great markets and restaurants. It easily can fill a day or more.

I think trying to do all 3 cities in 1 day from Milan would be rushed. If you decide to attempt it, I would take a Freccia to Bologna, explore in the morning, have lunch at Tamburini (famous deli that serves inexpensive and delicious lunches) and then stop in Modena and Parma on the way back. Some people don't seem to like Bologna, but I really enjoyed my stay there.

I would not take a car since the train is so convenient and inexpensive. I’ve not visited Modena but did visit Bologna once and Parma twice. I found Parma to be a nicer city to visit than Bologna- more manageable, more interesting sites, GREAT food. Bologna was okay but to me seemed nothing special compared to many other larger cities in Italy.

if taking the train, are those cities walkable -- particularly bologna?

In Bologna, you walk about 15 minutes to Piazza Maggiore. The historical district is around there, including the Neptune Fountain, the food market streets, the cathedral, the two towers and the information office. Also, you see many of the wonderful porticos.

I was in all three this past summer and think all would make an excellent day trip. The city centers are all accessible on foot (or bus) easily from the train station. They aren’t included in the Rick Steves guidebooks, but they are in plenty others. Try the Rough Guide. See which one interests you most.

Highlights in Bologna include the cathedral and the university. Modena’s cathedral is unique, they have lots of balsamic vinegar tasting, and there is a Ferrari museum near the train station (the factory is further out). Parma has a
Renaissance theater, a beautiful opera house and a stunning cathedral.

Thanks for the suggestion!

Leave the car too many ZTLs do deal with.

If you pick Bologna don't miss the market in Bologna Mercato delle erbe ... great place for lunch. We met friends who are Bologna residents there... it had to be one of the best meals we had during our 3 weeks in Italy. Learn to love carpaccio. The library is a nice stop if it's raining. Bologna has more spires than any city in Italy and the porticoes are amazing if it's raining you can pretty much still walk everywhere.

I'd skip Modena (pronounced/ MO denna) unless you want to visit Ferrari and Lamborghini both of which were surprisingly interesting we did visit Acetum for a private tour of the new facility...great Balsamic Vinegar!