Another line of thought.
When the soundwaves arrive the soundwave has a definite form which will be slightly different at the three points ABC. The essence is that the peakvolume of the soundwave is proportional (linear, quadratic or otherwise) with the distance. The farther away the weaker the sound. As one could calibrate the micro's with a signalstrength/distance table the math would become simpler again.
Don't know if the differences are within the noise-level of the signals/ micro's/arduino ADC but a simple test could reveal this. Furthermore soundwaves can be deformed by obstacles in the open field etc. Still it has some potential worth investigating.
Question: How big is ABCD in square meters? smaller/bigger?
Some additional hyperbola math: - http://www.codecogs.com/reference/maths/geometry/coordinate/hyperbola.php