I'd be surprised if you need anything special at all. In the scheme of things, the PWM signal is very low frequency and I'd think the impedance of the signal input on each ESC relatively high. I've run higher frequency digital signals (800kHz vs ~300Hz) over 3-wire ribbon cable over similar distances with no issues. Decent cabling/termination and *maybe* shielding should be enough, but the empiricist in me says test it ;-)