It's not just a trigger you need, frozen. I'm assuming you have the terrain for your different arenas, though.
1. Make sure you have a camera for each arena, as something of a preview.
2. The mini-map terrain should be hidden. If you want, you can even import a solid-black in-game minimap; however, forcing a UI key is easier.
3. Make regions for each arena.
4. Make sure you have an unused rect/region variable.
5. Use the example from my triggers from my in-progress map. You can use different methods than arrow keys if you wish. Make sure that only the host/player1/person-of-power can select arenas. (Don't let the "Number of players" crap confuse you; it's just a system of recommending the vastly differently-sized arenas that i used)
6. I suggest you set the camera bounds to the arena you picked, so that the players can't "visit" the others. My map does not do this, but yours probably should.