7565291
I'm tempted to say it'd be an apex predator, especially in the ocean, and be able to take down a T. rex if it dragged it down to the depths (I'm assuming by its anatomy that it'd be a slowpoke on land), but a lot of said instances of multi-headed creatures IRL often die at an early age, due to the heads operating on different goals and one body, or sometimes even attempting to eat the other head. So unless two of the heads are just puppets (in a way) controlled by one main head, I find it hard to believe it could survive to adulthood.
That's just my two cents on it. It could be a totally different case for someone else.

7565291

do you think that it would be able to stand up to the Tyrant Lizard King, AKA the T-Rex?

Unless it would be way bigger, no. That creature's jaw is obviously not up to the standards of raw power that the T-Rex had, and its necks are too slender. The T-Rex, if similar in size, would just pluck the heads off one by one.

Also, it has flippers. Its aquatic so they wouldn't meet.

Also, the heads would definitely argue who gets the eat the prey (assuming this guy swallows prey whole and doesn't tear off pieces when eating), leaving it vulnerable to literally anything large enough to even think about taking it down.

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also the left head (right from our perspective) is giving me major All Tomorrows Snake People vibes.

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Looks cool. Reminds me of a sea hydra.

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No, this guy rips its prey to shreds, it doesn't swallow them whole unless they're too small to give all three heads enough food, so each head could easily swallow fish that would be considered a light snack, but if they're eating a full meal, the three heads rip it to shreds to share. And IDK about the fact of the two heads being puppets, I was kind of thinking that its brain isn't in any of its heads, so in a way, none of the heads have total control

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interesting. That would up its chances of survivability. However, as Crimmar states, this creature seems aquatic, and unless it catches one off-guard, it probably wouldn't stand a chance against the king of Hell Creek.

7565395 No animal has ever starved to death because one head wasn't enough to swallow food. Just look at yourself. If you only had 6 minutes per day for eating, that would be plenty of time for survival. Having 3 heads to lower the needed time to just 2 minutes wouldn't present any benefit. After all, in reality, you have all day long for eating. And besides, a head wastes 30% of the energy. Why would you want to add 2 more?

There's a reason why 3-headed creatures aren't a thing in nature. There's no survivability advantage in having 3 heads.

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By up, I meant slightly. I should've specified, so that's on me.
Also, the heads would probably still argue who gets what piece, since two or all of the heads may consider the same piece as the best piece. Once again, they'd be vulnerable to some similarly-sized carnivore, or, if they were REALLY unfortunate, they'd be on land arguing which piece to take before heading back to the ocean, which would leave them vulnerable to, say, a T. rex.
Then again, that slight chance might be nonexistent, due to the new info I gathered from your comment. This is why single-headed creatures are easier to speculate about.