You are right, I drew it out and basically the 3rd sensor (lets assume it is at the very front out at x distance) should recevie a signal either before (if the source is dead ahead) or between the time that the left and right sensors receive it. If this is the case the the source is in front, if it receives it after the other two then it is behind.

Or something like that anyway, I just have to test it now.

Thanks again for all the ideas