Keep in mind that the 3 to 1 principle is not a RULE. It is a basic guideline in order for the engineer to visualize the movement of sound through the atmosphere and into your microphones.
darrylm mentioned reversing the phase of one of the microphones. This can be done to help align the phase of an phase-offending microphone. If you have 2 mics that cancel each other, 'flip' the phase on ONE of the microphones, and this may often help align the phase of the two mics.
For instance, many times engineers will place a mic 1/2" from the top of a snare drum, while also placing a mic under the snare as well. Often it may be helpful to invert (flip) the phase on the bottom mic.
Most quality microphone preamplifiers have a phase-invert button on them. It's often designated by a symbol that looks like an "O" with a diagonal line through it. You can try flipping the phase or moving one of the mics a couple of inches and it usually clears up any phase problems.
There are many useful visual phase meters which let you "see" the phase relationship of 2 or more mics. Meters such as the
Waves Dorrough Meter. It's often better to use your ears, especially if you're just starting out.
Be careful recording acoustic guitar or a guitar cabinet with 2 mics. Also, if you're recording a bass amp while taking a direct signal at the same time, you almost always will need to reverse the phase on one of these signals.