So, with no real information to go on, I’m going to take the assumption that the first two Ones are set up as surround speakers, bonded to the Arc. That means the third Sonos One can only be set up as a separate room, which means you have to group the two rooms, in order to get the same content playing in both rooms. Note that when the Arc is playing the TV input, there will be a slight delay between that room, and any grouped rooms, but when not using the TV input, streaming music, they’ll be in sync.