Board Game Arena

I'm in a 3p game. Turn based. Me and another player have made the first moves. The 3rd player never made a move. I can expel him now. I know it turns into a zombie and the game is played out without ELO.
But can I end the game then? I don't want to play a 2p game with 3p setup.

So what can I do to finish the game? Expel him and then quit the game? Will I get a leave penalty?

You can expell him, then propose to abandon the game collectively (in the "leave" menu, you have several choices). Then, just wait for the other (non-expelled) player to accept, and the game will end without any penalty for both of you.

I expelled him and the game was auto canceled (it was started as a minimum 3p)
I guess when it remained only 2 players and a zombie, it cancels by itself with no penalties for the remaining players.

It depends on the game. Some automatically stop and some continue, but those which continue will "not exist" - i.e. not show in your games played, no ELO, etc. You'll just be playing for fun. If the game continues after expelling someone, then you can deal with the option to abandon.