Since the connection is above the flood plane of the fixtures below, you can use a santee.
I have 3" and a 1 1/2" horizontal vents that i want to tie together and go vertical out the roof.
This connection will be made in the ceiling before going out the roof.
I thought about using a combo wye/tee or sanitee fitting but those fittings would only slope back to the vent in one direction. Doesn't it need to slope back to both vents?
A two way clean out fitting is the only thing I know of that would accomplish that.
ABS, and PVC, fittings do NOT have a "fixed slope" to them. Once you apply the cement you can "bend" them as necessary to get the slope. I do not know how you intend to make the connection or how a "two way cleanout would work because you would have to use bushing to reduce to the branch sizes. Almost any conventional fitting will work, but you may be thinking of a "vent tee" which is "square" without any "sweep" in either direction.
Licensed residential and commercial plumber