What protocol are you using to talk to the ESCs? I recall that on CC3D, OneShot ESC protocol has special requirements that move the RC receiver input pin around among other things and require using a different RC input mode in the setup for the motors to work.
You say the motors don't even work with the slider. I assume you are talking about the Output page sliders and that you have enabled the test outputs check box at the bottom of that page and that you have done this with PWM and it worked.