Cluster of two nodes proxmox 4.4

As a requirement I had read to achieve high availability (HA) I needed 3 nodes, I never understood why 3 instead of 2
 
As a requirement I had read to achieve high availability (HA) I needed 3 nodes, I never understood why 3 instead of 2

because you need quorum to avoid split brain.

think of network failure between only 2 nodes, each node will try to restart the vm.
 
Here I now have 2 nodes and circuits, for high availability like enough.
I can make a cluster without HA, And after adding the third host to include HA?