Hi,
I'll chime in with.....You do know to check the fluid with the engine idling?
Happy Motoring!

 

Sounds like you might need another tranny. The 3 speed c4's were very popular in drag racing back in the day if I remember correctly. Check your fluid level, if you're good on fluid and it's still revving between gears it sounds like it should be replaced, as others have stated. Find another, rebuild it and make it stronger, keep fluid up, and enjoy.