Keep breaking 3rd gear

I have broke 3 transmissions in 2 years. They are 18 spline 833 4speeds. It is always the 3rd gear, on the countershaft. It usually does not hurt the 3rd drive gear. This last time it did thou. They are all in street cars, making about 720 hp/660 tq. I don't speed shift, or have sticky tires. It always happens when it hooks in 3rd gear.

I know everybody is going to say get a Lenco, but I cant cut the cars. Is there anything I can do to help? Would the gear oil matter? I use 80/90 gear oil now. Is there anything you can do to the gears to strengthen them? I will be buying another countershaft, and drive gear tomorrow. Can you send them out for treatment? This is getting expensive, and the counter shafts are getting harder to find.

Lastly, I see they make a Chinese countershaft frown anybody have any luck with them? Harder to find originals anymore.....