Many of the tub manufacturers make an add-on tiling flange that can be used on a drop in tub for your situation. I think you're going to have trouble finding one designed for two people that is only 36" wide. Lots of brands out there, but I ended up with one from www.jasoninternational.com. ANohter technique that works is to use a surface membrane and use a urethane adhesive to adhere it to the tub surface and use an expansion joint. Look at Kerdi, KerdiFlex, and Dilex at www.schluter.com.