you to have Right click on Cluster and select Add Host----Type the name or IP of the host need to be added----it will further ask you to add the license key for that particular Host---Add the license key and finish to add the host.
if you created it as a 2 node cluster, there is no way to later convert it to a 3 node cluster.
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In VSA, you can create a 2 or 3 node cluster, but once created you cannot add or delete a node.
For more information, see Unable to add or remove VMware Virtual Storage Appliance nodes/members after installation .