I’m pretty sure your problem is with the overrunning clutch. Putting it in “low first” applies the rear band, which does the job of the overrunning clutch, that’s why “low first” works when normal “drive first” does not. In third gear, the front and rear clutches are engaged, and the overrunning clutch is not needed. In reverse, the rear band is also applied, which explains why reverse still functions. Unfortunately the transmission would have to be completely disassembled to repair that.