I am installing a new tub, utilizing the existing plumbing. The floor is concrete slab. I have 3 walls surrounding the tub.My question is how will I be able to hook up my drain? I believe that once I put the tub down, I won't have access to the bottm of it. I am assuming the previous owner did this while the one wall was not up yet.

You have no choice. Unless your tub has a removable front apron you will have to cut a access hole behind the drain to hook up thr tub waste and overflow to the trap. As a rule we always have the dry wall men cut in a access hole about 14 X14" square. Then the trim man makes a frame and panel and the painters complete the job. Look real hard behind the tub, it might just be tucked away in a closet. Good luck, Tom

Thanks! There is an access panel!