Installing a drain for a new tub

Installing a drain for a new tub on existing slab

I am installing a jetted tub in an existing bathroom. I have the water lines and electrical run to the tub deck area. What is needed to tie into the existing drain line. I have several to choose from. 1 shower drain ( the furthest away ) 1 sink drain ( the closest ) 2 seperate toilet drains ( 1 is much closer than the other). This is on an existing slab floor. I am aware of the concrete saw and the jack hammer that are necessary. This is an older home that I am remodeling. The existing plumbing is galvanized, the drains are cast iron. Is there a way to tie in the new drain going pvc to cast iron. If so what is the easiest way?