My headcanon for Owen not remembering 3-PO is that
- The odds are so astronomical that 3-PO would show up at his doorstep 20 years later in a Jawa sandcrawler that he doesn’t even consider that it could be a droid he once owned
- most protocol droids sound similar (TC-14 sounds prissy too).
- 3-PO is a different color to when he owned him
- He doesn’t even have the droid for any period of time before he before dies (as in, maybe he would have remembered eventually).
Point 1 is the big one though.