I made a triple PWM motor controller with a 555 timer hooked up to 3 IRF621 MOSFETS to the motors. I am having interference troubles. When I turn one knob up, the other motor wants to change speed a small amount, and it is not stable. I added a 47pf capacitor from pin 5 to ground on each, but that didn't seem to help. Is there a way around this, or is that just how the 555 timer is? Here is the schematic I made (yes I know it is a very bad schematic).
I notice from your circuit that the 555 and motors are running from a common (5V) power supply without any form of decoupling. The 555 is a current hog and draws a surge of current from the supply every time its output changes.
Try putting a large electrolytic (say 220uF) and a 0.1uf (helps with high frequencies) across each the Vcc and ground pin of the three 555s.
I would also make the capacitor at pin 5 a bit bigger (say 0.1uF) - its job is to smooth out any noise of the internal divider chain of 5K resistors. 47pF is way too low.