I'd recommend 4 nights if you're planning on taking in a Bologna match (I found it quiet a lot of time-consuming fannying about to get tickets; go to small ticket-selling bank branch in main square in one of the few hours that it's actually open, join long slow moving queue, passport inspection...
Lovely city with more neglected art than you can shake a stick at.The main square has an impressive statue of Neptune in the middle, not sure why. There's TWO leaning towers! You can walk a fair way through the town under porticos (not unlike our own Madeira terraces) which is handy if it rains...