Co-Op games for three players?

If you guys like rogue-like RPG, Barony it's nice [It's kinda FPS, tho]
There's Gauntlet too, if oldschool isometric are your cup of tea

https://www.gog.com/game/warlords_iii_darklords_rising when my family was alive we play this togheder and we like much , but we play the first n1 , this is 3 , but anyway is fun to play togheder.

Not really lesser known, but:

The Trine games are specifically 3 player coop focused.

Terraria can be also enjoyed in coop with 3 players (not on small map tho)

Also Don't starve together, though it is not everyone's cup of tea

Trine series

Warcraft 3.
True, I think, since now there is only a reforg (he changed the graphics a lot and increased the load), you will have to look for the old version and play via LAN

Tabletop Simulator might be worth a look. Thousands of free games in the workshop you bound to find something for you all to play.

https://store.steampowered.com/app/477160/Human_Fall_Flat/ - this is pretty fun
https://store.steampowered.com/app/641990/The_Escapists_2/ - this is fun as well
https://store.steampowered.com/app/251170/Damned/ - this one is horror, but was really fun

I think all of them should work on older systems fine

i've also been playing Unrailed, it's highly addictive
it adjusts speed to the number of players and it's doable with just 3

https://store.steampowered.com/app/632360/Risk_of_Rain_2/
https://store.steampowered.com/app/394510/HELLDIVERS_Dive_Harder_Edition/

Should work on lower end as well.

Please, don't think that i'm joking. Try this - https://store.steampowered.com/app/673750/Super_Bunny_Man/
The game is hilarious, you'll be dying of laughter some minutes in

Left 3 Dead

https://store.steampowered.com/bundle/575/Sid_Meiers_Civilization_V_Complete/ (DLCs are must)

https://store.steampowered.com/app/269210/Hero_Siege (It gets high discounts and has 4 pack)

https://store.steampowered.com/app/274190/Broforce/ (free version is: https://store.steampowered.com/app/312990/The_Expendabros/) I know this is not what you asked for but it's stupid fun.

https://store.steampowered.com/app/40970/Stronghold_Crusader_HD/ (High discounts also)

https://store.steampowered.com/app/269030/The_Yawhg/

https://store.steampowered.com/app/382050/Eon_Altar/

https://store.steampowered.com/app/252110/Lovers_in_a_Dangerous_Spacetime/
https://store.steampowered.com/app/375480/Chronicon/
https://store.steampowered.com/app/239070/Hammerwatch/

heave-ho
death squared
tricky towers
endless legend
golf with your friends
barotrauma
castle crasher

https://store.steampowered.com/app/512900/Streets_of_Rogue/
Fun RPG game that will run on even older PC. Specially fun with friends. Support Remote Play Together.
https://store.steampowered.com/app/563560/Alien_Swarm_Reactive_Drop/
May not run on older PCs.
https://store.steampowered.com/app/933110/Age_of_Empires_III_Definitive_Edition/
You probably know about this.
https://store.steampowered.com/app/17480/Command__Conquer_Red_Alert_3/
Both 2 & 3 will run.
https://store.steampowered.com/app/226840/Age_of_Wonders_III/
Similar to Civ.
https://store.steampowered.com/app/261180/Lethal_League/
Arcade Game.

Will add more if I remember them. I had that problem too. Most of them are already listed in other comments.

Heavy Burger https://store.steampowered.com/app/922250/#app_922250

You want a really obscure game for up to three players?

Threat. Old shareware game from the mid 1990s. It's a top down perspective game kind of like Smash TV. Hotseat multiplayer only.

https://www.youtube.com/watch?v=kndhStFG-yg

Wow, thank you all for the suggestions. Quite a few I've never heard of and look really good! We'll definitely find a few all three of us wanna play :)

earth defense force ... well its a 3rd person shooter though :P
warframe (free) 3rd person... hack n slash shooter
starbreak (free on steam \ webbrowser game) massive up to 40? players in one instance platforming rush to the end n kill the boss of the level.
helldivers (top down shooter)
alien swarm (top down shooter, free on steam)

Gauntlet like is... Forced (i think it has a longer name) but its by the makers of Minion Masters

Broforce :D although it has one level that you absolutely need to only have one player play it. literally impossible otherwise

Zero-K free RTS game on steam, one of the best ive ever played. it has the supreme commander look n feel.. ish

turn based? literally any of those advanced wars \ fire emblem like games ever. Which reminds me of WarGroove. a nice gem of a game, but like every game in this genre, can be hampered if a map set up is like a mirror = stalemates until one side forces a breakthrough)

guacamelee

These worked well in my 3 player group (note: none of these is turn based and they are mostly action focused):
-Rayman Origins/Legends
-Castle Crashers
-Monaco
-Overcooked
-Terraria

Also heard good stuff about Towerfall and Lovers in a Dangerous Spacetime, but haven't tried them yet.