Ouch!
Presuming this is their 2.95 gear kit it appears that you need the 3-4 half of the cluster gear P/N GFT502 as well as the third gear P/N GFT 504.
This generally happens when too much torque is applied to third gear. Either by too much raw power, or from using too strong of a clutch that bangs the power to the transmission on hard shifts.
So, how did it happen and how much power are you pushing the transmission with?
I know that G-Force has discontinued lots of things. These gears may not be available.
I know who to ask. I will see if I can find out.