For both sites, it depends on how much art and museums that you wish to see. If you like a lot, you will barely make it as there is much to see. As for Pisa, there isn't much to see outside of the leaning tower and historical grounds next to the tower. I suggest that it's a 3 hr venture into and out of Pisa, including the walk in and walk out. The train journey will be about 3hrs roundtrip. Thus, it's a 6-7hr. day trip.
For Venice, I would suggest the Accademia (not like the one in Florence), the Peggy Guggenheim museum, the Doge's Palace and St. Mark's Basilica. Other times can be spent on the vaporettos and getting slightly lost walking around.
For Florence, I would suggest the Accademia to see "David", the Uffizi (about 2.5hrs.) and the Duomo Museum (behing and across the street from il Duomo) for starters.