Valheim

Can vouch for Deep Rock Galactic, Ember Knights, Ultimate Chicken Horse, and Factorio

Edit: Pulsar: Lost Colony is also good, but you'll need to have two bots as well

Left4Dead2 can be played by 1-4 players. Empty player slots are filled by AI bots.

Lovers in a Dangerous Spacetime is fantastic.

Astroneer, Raft, and if you like mindless indie stuff Generation Zero.

Remnant 1 & 2 if you like souls type third person shooters.

Dungeon of the Endless and the new version Endless Dungeon.

For the king. Made for 3 people

Unreal Tournament 2004, with few players it turns its normal frantic and fast paced gameplay into a suspensfull slower experience which is awesome.

I think you can get it on the internet archive as it stopped being sold on gog and steam this spring.

OpenRA - Do you remember the old classic series Command & Conquer ?

OpenRA takes the old original game, updates the controls to modern RTS controls, add new ai to play against, add support for modern resolutions and adds a good internet multiplayer system, all of this for free!

OpenRA can be downloaded att https://openra.net

I haven't seen it mentioned, but Minecraft is also multiplayer.

Deep Rock Galactic!

Trine 2! It’s a side-scrolling action/puzzle platformer, and a rare game actually made for 3.

Nobody mentioned the finals. Free to play fps competitive shooter with teams of 3. It's been crazy addictive for my friends and I. If you like fast paced hectic fps games it's loads of fun.

I'm in a poly throuple and we play these games:

-League of Legends

-Super Mecha Champions (It's like Apex but with bots and the real players aren't as good so we don't get absolutely crushed every time we play which is nice)

-Stardew Valley

-Eternal Return (League + Battle Royale)

-Divinity 2

-Overcooked

-Borderlands

-FF14

-Tabletop Simulator (Board Games)

Online or offline multi-player?

As for couch coop Rayman Origins and Rayman Legends is some of the most fun I ever had with friends gaming.

Sea of Thieves

I didn't see anyone mention Stardew Valley. My two kids and I put in 100+ hours on our farm. It became a nice, zen, pre-bedtime ritual.

Remnant 1 and 2 are both 3 players

Some of my favorites for 3 are the same for 4.

Streets of Rogue and Vagante are challenging roguelikes that scale effortlessly for co-op of various numbers of players. There's also Mercenary Kings, which I admit I enjoyed more than most, but it's like the cross section of Mega Man action with Monster Hunter structure. Much more recently, Baldur's Gate 3 probably hits its sweet spot at 3 human players, since you want to have an NPC slot around in your party of 4 for the story quests that are relevant to that NPC.

Monster Hunter

If you're into RPGs, both Solasta: Crown of the Magister and Balgur's Gate 3 are great coop experiences.

Nine Parchments and Magicka 2.

Edit: also, the Trine games. And on PlayStation we enjoyed Diablo III and Alienation, both of those are three player as well.

I mean, love it or hate it, Destiny 2 is literally made to be played by (mostly) 3-person teams. I’ve had a boatload of fun with it over the years.

If y’all have PS3, Lord of the Rings: War in the North. Specifically a 3 player co-op game. Like actually 3 player, not 4 player where you don’t need a 4th. I enjoyed it and have good memories playing with friends, it’s really fun and silly and the servers are still up.

Some beat them up: tmnt, streets of rage 4

Also since you did not mention playing remotely or not (although there are ways around that: parsec, steam remote play together...) almost any couch coop game: overcooked, moving out, unrailed...

Edit: all of those I cited have a online multiplayer mode I think, for overcooked the AYCE edition contains all of the first edition content but brings online MP amongst other things

A couple older games:

Supreme Commander: Forged Alliance. A very complex RTS with a pretty steep learning curve and advanced strategy. Awesome game but not casual.

Alien vs predator games are specifically 3 player (marine, alien , predator). Not sure if there is a modern version. Interesting because each of the 3 play differently.

I play with 2 of my brother in laws. These are my favorites:

Elden Ring

Dark Souls 3

Borderlands 3

Ghost of Tsushima

Rocket League

Avengers

Remnant

Crawl

Stellaris allows many players if you like those types of games. Only the host needs dlc if you want to pool your cash and buy some. But yeah it can be expensive.

Towerfall Boomerang Fu Hammerwatch Splitgate Quake Live / Champions

Fortnite supports trios.

My wife, my brother, and I like to play Hunt: Showdown.

Elden Ring with the seamless COOP mod has been very fun for my two friends and me. The experience isn’t 100% perfect (some bugs) but exploring the Lands Between with buddies and fighting the bosses together has easily been in my top 3 COOP experiences up until now.

OP What games did you guys end up playing or selecting? Also what platform are you playing on?