if both guitar mics are out of phase with the vocal flip the polarity on both guitars tracks, it will keep those in phase and should put it closer in phase to the vocal mic. How are the guitars panned? are they hard left/right? Was this a dual acoustic with live vocal scenario? Is there any acoustic treatment in the room? Did you want vocal bleed in the guitar mics?
The general thought is if the mics are more than 1 meter or so apart then the possibility of phase issues are less likely.