OK, what do we notice?
First of all, it's CR three. And it's a bruiser. But it's also a fast bruiser - land speed of 40 and has a swim speed. It's got real bruiser status - large size and a 13.5 point attack. Two 13.5 point attacks.
Secondly, unlike other CR 3 bruisers, its relatively weak Will save means precisely dick, because it's a Vermin, and immune to anything a 3rd level party can dish out with a Will save.
Finally, we note that it has improved grab and a grapple bonus of 19. Also, it has 66 hit points and an AC of 19.
So how does combat with it work out? Like this, every damn time:
Round One: Giant Crab charges 80 feet and reaches out and touches someone (10' reach) for a +12 attack bonus. Then it does 13.5 damage. Then it Grapples, which it wins, and inflicts another 13.5 damage of constriction.
Round 2: Having just inflicted 27 points of damage on a 3rd level character in one round, it hurls the bloody carcass over its head and repeats the process with some other hapless 3rd level PC who happens to be within its 90 foot charge range.
Round 3: Having dispatched 2 PCs, it takes out the third PC.
Round Four: Assuming that the final character has not gotten a rush of brains to the head and hid, the Crab finishes off the last player character and dances around.
Motherfvckers! What the hell CR mismanagement is that shit!? This isn't one of those "It's a huge ass scorpion, dumbass, don't melee it!" problems - this is CR 3. You don't have any choice.