Soldering very loose 3" drain pipe possible?

[wwhitney]

On the way to the main stack tub connects a sink drain, the sink drain has it's own venting above the toilet tee.

Yes, the lav drain can wet vent the tub drain in that fashion, so the wye at the stack is fine, and perhaps preferred.

Note that the tub trap arm, from the trap outlet to the wye where the lav drain joins the tub drain, should fall at most 1 pipe diameter, while falling at least 1/4" per foot.

Cheers, Wayne