I cant remember anything about CDF's Please can someone help with this question!!!

Obtain the cumulative distribution function of the following discrete random variables

(A) Bin(3 , theta)

(B) Unif( 0 , m)

(C) Geom( Theta )

The question did give hints but they don't help me what so ever

HINT

First calculate F at the integers. (Distinguish the index of the summation from the upper limit of summation). Secondy extend from the integers to the real line.

Any help would be appreciated!!!