Well first off, which school has professor(s) or specialities you most want to work with?
Second, how do the financial aspects of your offers stack up, and how important is this to you? (In general, the cost of living is probably cheaper at UGA than the other two as well)
Third, the math grad depts are ranked by US News as follows: UConn #87, UGA #52, UPitt #60 out of #134 (there are several ties on this list, though).
Fourth, location. UConn is east of Hartford, CT and just over three hours northeast from NYC. UPitt is also in the northeastern part of the country, located in Pittsburgh, PA about 5hrs northwest of Washington DC. UGA is much further south, located in Athens, GA about one hour east of Atlanta (all times are estimated driving times). Does this matter? What kind of place do you want to go to?
Now as a student of UGA, I am obviously biased because I love living in Athens. But I think any of these options are a great choice, it all comes down to what points you decide are the most important, and which of the schools fit you best.