You can get drop-in tubs that have a tiling flange on three sides. The outside rests on deck wall you build and tile - but water doesn't get on that outside lip - because of the shower door or shower curtain.
You need a tile flange. What's done is to shim the studs above the tub (say 1/4") this allows the cement board to come down over the flange. You caulk this space with 100% silicone.