Hello ameet009,
Welcome to Communities,
As Nicolas said, it is just a case of 'drag-and'drop' and configuring the necessary functions - vSAN Health will aid in validating some (but not all) of the possible things that could be misconfigured in doing this.
From the fact that you mentioned that you are running a 5-node cluster currently this indicate that you are running FTM=RAID1,FTT=2 (as minimum is 6-node for FTM=RAID6), thus adding more nodes and changing the to FTM=RAID1,FTT=3 will essentially just create an extra mirror (and and extra witness component) on the new nodes (and not result in a 'deep-reconfig' of all data as it would if for instance changing from RAID5/6 to RAID1).
More to the point though, can you tell us some of your design justifications as to why you are considering (from the way you have phrased your query) running FTM=RAID1,FTT=3?
The vast majority of vSAN customers run FTT=1 and a portion FTT=2 and then in addition to this go with the more modern design decisions of redundancy at depth (e.g. failover sites, in-guest clustering, application-level redundancy and load-balancing).
If you are also doing these things here and just want some extra fault tolerance in a priority cluster that is fine, but if not then do consider that designing any infrastructure should never be just about one cluster.
Bob
