I can't see how a pack of hyenas would benefit from binding their bodies together physically, not to mention I'm not entirely sure how they would do it if they wanted to. Flanking is much easier if you're not going solo. But maybe, your puppies didn't have the choice? Having your puppies be conjoined triplets solves the motivation issue nicely, as well as the binding method, and it makes muscle coordination easier. Specifically, your Cerberus have a medical condition known as polycephaly. Yes, I know, you wanted separate dogs. They are separate personalities. Linked from the birth? To a T.
Can they even survive? Wikipedia says
In cases where multiple heads are fully developed and non-parasitic, they share control of the organs and limbs, though the specific structure of the connections varies. Animals often move in a disoriented and dizzy fashion, with the brains "arguing" with each other; some animals simply zig-zag without getting anywhere. Snake heads may attack and even attempt to swallow each other. Thus, polycephalic animals survive poorly in the wild compared to normal monocephalic animals.
Most two-headed snakes only live for a few months, though some have been reported to live a full life and even reproduced, with the offspring born normal. A two-headed black rat snake with separate throats and stomachs survived for 20 years. A two-headed albino rat snake named "We" survived in captivity for 8 years.
So ... not a great prospect. But that's only because the two heads are thrown into an unfamiliar (for their genetic programming) situation they can't cope with. A little education before they kill each other goes a long way. And you know what? I don't know about hyenas, but wolves are excellent learners, so I'd recommend those.
The first generations of cerberids would be tough, but the declining quality of genome in the local pack made sure the condition would prevail nevertheless. They would learn to live with three heads because single-headed puppies were getting too rare. Untrained puppies would meet their untimely demise due to fights between their heads, but the second triple comes through, their parents already know to keep an eye on them and break up fights. And of course, polycephalic parents already know how they were raised, and they'll use the same method on their puppies.
Wolves need clear hierarchy, and conjoined heads do even more so. But you can't have them establish dominance via death matches. Instead, the parents would make sure to always give preferred treatment to the middle head. The other two would then mostly just sit and watch until a brawl crops up.
Can they outperform other packs when they eventually encounter them? I can see some benefits of a well-trained trio of heads:
More eyes. Birds have to cope up with a tough decision. Either 360-degree vision, but 2D, or 3D vision but they can't see behind them. Your puppies wouldn't have the issue. The cohorts would look what's behind the body, at an angle of 120 degrees from the main head (eventually made easier by proper genetics). Then, when the call to fight comes, they'd swing forwards, sacrificing field of vision for ...
More teeth. Three sets of teeth are better than one set of teeth. Three sets of teeth in three separate bodies would be even better, but you gotta do what you gotta do.
Backup heads. When a monocephalic animal loses their head (literally), it's game over, man. When cerberos loses their head to a foe, it's not much worse than any other gaping wound in the middle of a deathmatch. Pretty bad, but survivors do pop up. Then, you have a
two-headed wolf with a nasty scar on their neck a pair of conjoined wolves forced to carry along a grisly reminder of their long lost friend - or, more likely, a long lost frenemy.
Plot hook: two betas, taught by generations to always listen to the head that sits between them and always commands them, are now stuck in a single body with no command and with a fresh memory of a fight they barely survived. Tradition says the right hand should take over lead, but the left head doesn't like the idea...