Does anyone have suggestions on must-see's during a 3-day stay in Milan?
Do you have Rick Steves Italy? He has a very good Milan chapter, outlining at least two days' worth of sights. The old center with the Duomo (be sure to go to the roof to see the gargoyles), the Galleria, La Scala, etc is a nice walk. And the Brera art gallery has lots of great pieces. I haven't been to the Monumental Cemetery, but it looks amazing.
If you do run out of sights in Milan, consider a day trip to Bergamo. A more typical suggestion is a day trip to Varenna and Bellagio on Lake Como, but I feel that is better as a stay of a few days rather than a day trip - your call.
Except in the case of dreadful weather, I'd definitely include Bergamo if I had three days in Milan.
Duomo, Sforza Castle, The Last Supper by Da Vinci, Parco Sempione- plus lots of shopping and great restaurants as well.
Happy Travels✨💫
Another vote for a day trip to Bergamo (Città Alta section). Also Basilica San Lorenzo Maggiore and Basilica Sant'' Ambrogio, along with the Duomo.