Okay so he is my idea. Take the Hell Hound stat block as a base. Make the Hell Hound Gargantuan and scale the stats and damage appropriately. Next give it 3 heads, and because it has 3 heads it has advantage on perception checks that involves Sight, Sound, or Smell.

Since it has 3 heads it gets 3 reactions per round.

As an action Cerebus can use its breath weapon and make 2 attacks that can either be bite or claw attacks. If it can't use its breath weapon it makes 3 attacks that can be either bite or claw attacks. If a medium or smaller creature is hit with the bite attack it is Grappled DC X. That head can not take a reaction or bite another creature as long as it is grappling that creature.

As its attack a head can swallow a creature it has grappled. The creature is restrained begins to take XdX fire damage at the beginning of each of it's turns until either Cerberus dies or you are regurgitated. It you deal 30 damage or more in a single turn while inside Cerebus you get regurgitated 5ft in front it.

3 legendary saves are a must.

I'm very tempted to save each head has a separate breath weapon recharge.

I want this to be at least a CR 20.