Originally Posted by
Hawk Circle
In response to the OP's question, three hours is plenty of time to travel from/to the airport and see the sights you mention.
^^
It's less than 3 miles from the airport terminal to Piazza die Miracoli, where you'll find the cathedral and leaning tower.
It'll take less than 15 minutes each way by taxi, which won't cost the earth.
No reason at all why you shouldn't manage to do it in the time available.