The 3:1 rule/principle is the one of the most widely misunderstood concepts in recording.
I can say something like "3:1 rule does not apply to stereo miking techniques", and 33% will agree and 33% will vehemently argue, and 33% will have never thought about it.
see here:
Question about the 3:1 principle - The Womb
and
Tape Op Message Board :: View topic - Question On The 3:1 Theory
Really the bottom line is, check in mono for phase problems, using your EARS!