Eddie Brock
Normal human, albeit an extremely fit human, with the symbiote enhancing his strength.
Mac Gargan
Superhuman with the symbiote enhancing his strength.
I'd assume Venom III would win, but then again in comics, strenght isn't everything. I mean, Carnage could rape Spider-Man and Venom effortlessly, yet Brock/Venom has beaten Carnage before, so...
It seems like Eddie's strength was enhanced some even w/o the symbiote. In Seperation Anxiety he was fighting one of the other symbiotes one-on-one and he was putting up a good fight, without help from his symbiote. Eddie Venom crushes Gargan. Not only because he's superior, but also because of how determined he is.