See these docs:
vSphere 5.5 Documentation Center - vCenter Single Sign-On Deployment Modes
Multiple Single Sign-On instances in different locations: You can install the vCenter Single Sign-On nodes in this deployment in any order. Any node that is installed after the first node can point to any node that is already installed. For example, the third node can point to either the first or second node.
vSphere 5.5 Documentation Center - Joining a Linked Mode Group During and After Installation
On Machine 3, you upgrade to vCenter Server 5.x. After the upgrade, you configure Machine 3 to join either Machine 1 or Machine 2. Machine 1, Machine 2, and Machine 3 are now members of a Linked Mode group.