This solution obtains the origin of gunfire using three microphones.

http://mypeoplepc.com/members/jon8338/math/id22.html

I have posted this before but I corrected a mistake I made.  This correction 
fixes the problem of the source of the gunfire originating from either side 
of the plane of the three microphones.  There are two solutions.  A fourth 
microphone comparing times determines which side of the bull's horns the 
shot originates.

There's a link on the site to an Excel 97 workbook that calculates the 
solution for different inputs.