Why 3 node cluster for Automatic Failover?

Hi All,

I am a Newbee to Couchbase.
I noticed that in order to have an Automatic Failover, i should have a 3 node cluster.
It is written in all forums and in Couchbase whitepapers, but non are explaining why does It needs 3 nodes for automatic Failover to work.

As I understand all nodes are Active. correct ?
So why do I need a 3’d node ?

I’ll appreciate an explanation that I will be able to understand.