Cap kicks sand in Namor's face to blind him, DD trips him over, and Batman knocks him out with a nerve strike. Fight lasts maybe the 10 seconds it takes to do this.
Otherwise Namor wins.
The only way the team win this is if they can last long enough for one of them to get in with a nerve strike, probably with the other two attacking Namor to provide a distraction. Given that Cap and DD have both managed to land a few punches against Namor in H2H in the past, and that each of the team is skilled enough to do a nerve strike, this is feasible.
I'd give the team 3/10.