Yeah, the 3:1 rule simply states that if you have two mics picking up the same source, place them apart at least three times as far as the mics are to teh source itself. This helps prevent phase cancellation and a nasty reduction in the thickness of your sound. It's ultra ultra ultra-important to have proper phase alignment between multiple mics on guitar amp miking because you'll lose all that fat low-end beef if things aren't set up right.