Legendary Resistance (3/day). If Cerberus fails a saving throw, it can choose to succeed instead.
Keen Hearing and Smell. Cerberus has advantage on Wisdom (Perception) checks that rely on hearing or smell.
Multiple Heads. Cerberus has three heads. While it has more than one head, the Cerberus has advantage on saving throws against being blinded, charmed, deafened, frightened, stunned, and knocked unconscious.
Whenever Cerberus takes 50 or more damage in a single turn, one of its heads dies. If all its heads die, Cerberus dies.
At the end of its turn, any heads that died since its last turn regenerate and Cerberus heals 20 hit points.
Reactive Heads. For each head Cerberus has beyond one, it gets an extra reaction that can be used only for opportunity attacks. 
Wakeful. While Cerberus sleeps, at least one of its heads is awake.
Bloodied Breath. When Cerberus is reduced to half hit points, its fire breath immediately recharges.

Multiattack. Cerberus makes as many bite attacks as it has heads and one claw attack.
Bite. Melee Weapon Attack: +13 to hit, reach 5 ft., one target. Hit: 20 (3d8 + 7) piercing damage plus 14 (4d6) fire damage.
Claw. Melee Weapon Attack: +13 to hit, reach 10 ft., one target. Hit: 16 (2d8 + 7) slashing damage.
Fire Breath (Recharge 5–6). The hound exhales fire in a 60-foot cone. Each creature in that area must make a DC 21 Dexterity saving throw, taking 63 (18d6) fire damage on a failed save, or half as much damage on a successful one.