How do I convert Atlas rolling stock from 2 rail to 3 rail?

 I assume you are referring to the New Atlas O Master Line and Trainman models. If so the easiest way is to purchase the correct 3 rail trucks with couplers from Atlas. Then remove the 2 rail coupler box cover, coupler and the two screws and mounting box from the body. Remove the two rail trucks and screw on the 3 rail trucks and coupler. Atlas O uses the same frame and body bolster on both 2 and 3 rail models so the conversion is easy.

Now, if you are referring to the Atlas cars from the 1970's thats a whole different ball of wax