I need to install a 3" "T" in a horizontal waste line. I will need to drop into the "T" two 1-1/2" lines one from a washing machine and one from a sink. Should I use a "T" or a "T-Y"
Both of the 1-1/2" lines are already vented, I am only repositioning the fixtures and need a new drop point.

Tee-Y

Definitely a Y.

Use "Y" and to complete the turn ( if needed ) use one 45.
If your washer and sink drains are coming from opposite sides, they also sell Double "Y" with each 1 1/2" inlet pointing in the opposite direction.

Suggestion: use 2" drain from Washing Machine. 1 1/2" is too small to absorb all the water being discharge from W/M. Water overflow may become an issue.

Back to you

Milo