why wont my jcb 3cx go forwards?

Have you checked the transmission fluid and filter? It might be due to low oil level. Other than that, it sounds like your clutch pack is excessively worn.
I have a 1979 3CIII with Powertrain transmission which used to have a similar problem. Changing the oil and filter certainly helped but you have to remember that your machine is nearly 25 years old.

steve
December 2010