I DO NOT KNOW IF THE SNIPERS USED THIS DEVICE. However, I will mention it here because I do know that such a device was developed and tested for sniper use in the 1980s. I have a book that discusses its use and training and has photos of same in my firearms library. Really, in this day and age a device like this could me made very small, light and reliable, and wireless to boot.
It is a simple computerized firing mechanism that is attached to the three rifles. As the snipers get clear shots on target, each one holds his trigger down. If the shot goes away, the individual snipers release their triggers. If at any time, all three triggers are depressed, the computer fires the guns simultaneously. Simple as that. The system will only fire if the specified number of inputs are occurring at once. With this system, you could have 50 shots taken together on 50 targets by 50 snipers.