I think the best introduction to 3-cushion is still
Byrne's New Standard Book of Pool and Billiards, combined with a regular diet of videos of top players (actively - not passively- watched).
CaromTV has the largest selection by far, but
Kozoom and
Accu-Stats have some great matches as well even though their catalogs are smaller.
Frederic Caudron's book is very good, but it's expensive and not as useful if you don't read French very well.
Warning: If you start with a bunch of systems as a beginner you will likely develop a billiard blindness that will be difficult to overcome later. The problem is you immediately try to force every layout into something you can calculate, and usually end up overlooking patterns that are much more natural for a position just because you don't have a system for it.
Have fun and good luck!
Robert