Had a replacement winch in the garage for a while because I didn't take the time to figure out how I wanted to retrofit a 3-bolt winch on my 2-bolt trailer. Others have just drilled the extra holes, but I didn't really want to do that and didn't trust I get the holes square on the square tube. I read a couple of threads on other forums folks have used a washer and one bolt on the back and the winch failed (or the washer failed, then the winch broke). So I came up with an alternate way to spread the load - will report back the results.
Basically I just cut a piece of 1/4" x 1" stock down to 2 1/2" and drilled a hole dead center:
Slide this in the winch body between the two gear assemblies, fish a 4 1/2" fully threaded hex head bolt and flat washer into the plate, and then towards the back of the winch. You'll have to massage it a bit by angling the plate to get it to fit, but I figured out this is the max size that will fit in there. The plate basically spreads the load where the 2 bolts would have done that job. I was able to fit a 9/16" box end wrench in under the gear assemblies to hold the hex head while tightening. I think this will be an effective and cheap solution for others that don't want to drill extra holes, but time will tell.