I have an existing 2" drain line roughed in under the slab in the area of the basement where I want to install a whirlpool, seperate shower, and vanity. If I open the floor and extend the 2" to each of the three fixture locations and properly vent each is the single 2" drain big enough for all 3 fixtures? The toilet is also roughed in already and we will be using setting the toilet where it is roughed in.

Hello Sririck, yes you can drain your lav, jac,shower,into a 2" drainline. with 1/4 per ft. slope.
A 2" drain will carry 14 fixture units, lav is 1 unit, tub is 2 units, shower is 2 units. -- (toilets must be at least 3"drain.) good luck----Zeke

Can't add much to Trucks great answer except to agree with it! Cheers, Tom