Yep, what Jorgen says.
On a recent United flight out of SFO T3, I thought I'd go look at the new T2.
No SFO T3-T2 connection airside.
What you CAN do airside is go back and forth between SFO T3 and the International Terminal.
(And when T3 security lines are long, even for elites, the Int. Terminal lines are often a much better bet, so that airside connector is handy.)