lowest drag for fixed 3 blade?
A fixed prop has about the same drag no matter who makes it. You can vary the diameter or the picth but thats all there is.
As far as locked or unlocked goes. It is more important to consider the transmission. My Hurth transmission requires the transmission locked in reverse so the prop does not freewheel under sail. Not all transmissions are like that so you need to find out. Some others can freewheel fine.
As far as if you can freewheel if it better I''ve seen it argued both ways. Since I have to lock mine I don''t care<g>.
If I had the money I would go with a Martec 3 blade. For now I lock the prop and just sail the boat.