Easyrtc uses a full mesh model on top of Webrtc, which means all the peers have connections to all other peers they are in a conversation with.
The limiting factors on how many people you can be in a conversation with is your upload speed and your processing power.
With decent internet connections, you should be able to do voice only (no video) with quite a bit more people than 3.
You may running echo problems though if people aren't hearing headphones.