Can a 60" x 30" x 79" shower/tub fit through 36 door?
Yes these should fit into any opening that is a half inch wider (tub measurements are for the outside).  However there are tubs that have a curvy outside shell or step-layers.  If your tub has something like this I would assume that extra width is not part of the tub width.  You can usually tell with a picture of the tub.

However we hardly ever bring tubs in via doors when doing a build.  We would simply just not nail in certain framing.  Then knock over the 2x4s, let the tub pass through (might do this for multiple walls and sometimes the only door we use is the front door), then nail up the framing after done.  This is just easier and keeps tub from being scratched up.