don't think on "refutation", just that you'll have a good position.
anyway, the key idea to know is that 3.. b6 4 b4 a5 5 a3 fails to axb4 and white can't recapture because the pawn is pinned (taking would run into Rxa1).
white's best is 4 cxb6 (which is basically admitting failure), and different moves just lead to different advantages for black.
It seems to me that after 1 d4-d5 2 c4-e6 3 c5, black should have a simple refutation but being kind on a beginner, I don't know what that would be. Can anyone offer advice?