Not exactly sure what the server limits are but something like Valheim could work for a large group of people. It's an open world sandbox so people can divide up and do whatever they want to on the same server (exploring, fighting, mining, building, farming, etc.) Not sure how far you can get in the game in 3 hours though..

Alternatively you could play some sort of team based shooter but there might be a stark skill difference in a competitive setting.

Apparently the star wars bf classic collection has a 32 player co-op mode vs ai which sounds amazing to me, but I've never tried it before