Monsters also generally get a cool one or two action ability, so they frequently don't just do 3 attacks. In the hell hound example, they get a breath weapon that takes one action. It could use its breath weapon for one action, move for one action, then melee attack at its full bonus. It's also not too bad at athletics checks, so it could spend an action trying a combat maneuver instead of just a melee attack.
They get three actions each round ...
Just as simple as that really.
And just because I think it is a good idea to link directly to the source where possible, Hell Hound.
Keep in mind that creatures with the "Minion" trait only have 2 actions
Very few DM-run creatures will have the "minion" trait. Unless some critters are animal companions, familiars or summoned creatures. The vast majority will have three actions, regardless of whether they have a single bite attack or two claws and a bite, or multiple arms or whatever.
Some creatures that have multiple appendages have special actions that take advantage of that. Like the Marilith which gets multiple reactions and also has several two-action special abilities which spell out how they can use their six sword-wielding arms to great effect.
Lacking any special abilities like that, all creatures have three actions, just like the PCs.
Yeah, minions actually have the opposite of their desired effect on the GM side. They make it harder to run your creatures, not easier. I have yet to see a NPC who uses the minion rules. Even the summoner I've seen gets an ability to sustain their summon as a free action to avoid complicating their turn decisions.