Sometimes it's pretty difficult to form a chord using all 6-strings or even 5 strings. I was wondering if there was a site that has a basic guide to forming chords using only 3 strings. For example, forming a Cmaj with 3 strings is simply:

e: x
b: 8
G: 9
D: 10
A: x
E: x

If I want to form a Cadd9, is it possible to use only 3 strings? Thanks, I need some of these shortcut formulas and thanks in advance.