The resolution cannot be set to 720p in a meeting of 3 or more people.

The resolution cannot be set to 720p in a meeting of 3 or more people.
In P2P mode, the resolution can be set to 720p.
Is my config incorrect?

It can be 720p for all participants no matter of the number, it all depends whether those participants have enough bandwidth to send or receive it …
Have you tried it on meet.jit.si ?

By this, I’m assuming you mean in Tile mode (in Speaker mode, the current speaker is always in 720p). In Tile mode, the client can request only 2 HD streams by default. You can change this by setting maxFullResolutionParticipants in config.js to allow clients to request more HD streams. You would also want to change the minHeightForQualityLvl thresholds to accommodate that change.