VXrail Stretch cluster

Hello, 

thanks in advance, 

we are trying to build Vxrail Cluster, we have total three sites, primary, secondary and DR, witness host will be at DR site. each sites we have Vxrail p570f version 7.0.241. i able to build cluster at DR site and able to installed witness host appliance. 

 Cisco ACI stretch cluster between two sites (Primary and Secondary), I have gone through so many documents but some of the things are not clear as mentioned below,

3+3+3 node cluster (Between Primary+Secondary+DR) DR host will be the witness host. 

1.i have power it up 3 nodes at primary site and 3 nodes at secondary site, do we need to discover all 6 nodes from primary site and build cluster?  or do we need to create primary site 3 node cluster and later we expand the nodes while adding secondary site nodes? 

2. can we discover primary and secondary site nodes (3 +3) from Primary site ? what is the right way to do?

is there any suggestion, am i on the right track?