That Damned Crab!

[Dausuul]

Level 3 fighter with a Con of 10 and 16-17hp?
I don't entirely think it's the crab that's at fault here.

That said, a CR of 3 for an aquatic grappler (Go! Super Drowning Skills!) with those specs is a little b0rked. 4 maybe. 3? Naaaah.

An extremely strong aquatic grappler. A level 3 character who's built for grappling might have a +12 grapple modifier--and that's with Strength 20, Improved Grapple, and full BAB. This thing has +19! And none of the usual anti-grapple tricks are available at third level. Once it hits you, it's got you and you're not getting away, and two rounds of being squoze in one of those claws will kill just about any third-leveler. The ability to dive underwater and drown you is just icing on the cake.

With a +10 to hit on its claw attacks, it's got pretty good odds of snagging two enemies on the first round. Game over, man.

Remember that a CR 3 should be an easy fight for a 3rd-level party, not an even match. An even match (50% chance for the battle to go either way) would be CR 7. This thing is a walking TPK.