My roughed in drain in a concrete floor is 15 inch from the stud wall. Can I install a 36 inch wide bath tub?

Hi FTB

You sure can. Here, when you install the PTRAP it should give you the additional 3" offset needed to accommodate a 36" tub (i.e., an 18" drain rough). If the PTRAP is already installed then if you have enough height between the PTRAP and the tub waste assembly you can use (2) 45 degree fittings to offset the vertical pipe to line things up...;)

Worst case, you need to cut out the PTRAP, install a 22.5 degree fitting and a new PTRAP and that should line things up just right!

Good luck!

Mark