Ive taken a look at the signal with my scope. At first it was showing a 1.4ms pulse with a 6ms gap. I found out that i hadnt cleared the Timer3 interrupt flag. Ive updated my code and now im seeing a 1.4ms pulse with a 19.2ms gap but its now worse than it was before. Originally the servos were constantly buzzing but now they are visibly moving constantly.

While ive been messing about with it ive been testing the digital servo on its own and it seems that it holds its position silently no matter how big or small the gap is. Its the 3 standard servos that are causing all the buzzing. I even plugged one of the standard servos into the socket for the digital servo just to be sure but i still had the problem