Well, that's your problem.
The SPT is 2.5" at the flange which mates to the DP. You need to get the 2.5" to 3" adapter, and get a new donut gasket as well.
For future reference, the SPT is 2.5" at that point, but buffs out to 3" just a few inches further down. Then it shrinks down again at the Y.