Tee or wye

Im curious about this situation, please help. and explain why

This is just an example, im just curious about the fitting nothing else

For a waste arm connected to a vertical waste pipe, you must use a sanitary tee, and not a wye or combo. The reason has to do with where the vent takes off with respect to the invert of the waste arm.
If you look at this catalog http://www.nibco.com/assets/plasticfit.pdf
compare the combo ( long radius TeeWye) # 5812-LR on page 31, to the san tee 5811-2 on page 25. You will see that if the combo was vertical, the trap on the waste arm could siphon, not protected by the low vent takeoff.

As Jimbo said, you MUST use a sanitary tee fitting here. See drawing below for a visual on what we are talking about, OK?

If you are worried about snaking the drain then install a TEST TEE fitting in the vent above the sanitary tee fitting if the test tee fitting will be readily accessible..

question about the same drawing, if its a 3 x 1 1/2 tee, how high do i have to go up before i can reduce the vent to 2"

It depends on what is above the sanitary tee fitting, if anything, and if there is already a 3" or larger vent pipe going out the roof of the house?

Also, why are you running 3" in the first place...what fixture is this we are talking about??

quote; how high do i have to go up before i can reduce the vent to 2"

That is an open ended question. You might be able to install a 3x2 bushing in the top of the tee, or you might have to continue the 3" all the way through the roof. The answer depends on several factors which we do not know since it could be a local requirement.

The tee/Y/combo requirement has NOTHING to do with a water to air ration, and EVERYTHING to do with the dynamics of a siphon, or "S" trap.