YES!!!, it is a MUST DO!!!, if you don’t replace the stock 3rd gear it will fracture, taking many gears along with it and probably the counter shaft with it. I should know, it happened to me. Took out 6 gears and the countershaft, I quit counting dollars when I got to $400.
The stock 3rd gear has cut-outs that go completely thru the gear, which makes it weak. The aftermarket gear has cut-outs for dog engagement on the side that is required for engagement and the other side is solid, giving it strength. Never had an aftermarket gear fail yet.
You can get the gear at sharkshifter.com, it might seem a little pricey at $112, but compared to $400+, and any other damage that could happen, it is well worth it.