Welcome to the MongoDB Community @BHARATHARUN_RAMASAMY !
I strongly recommend not doing multiple major changes concurrently (for example: upgrading major release versions and migrating to new hardware). If you encounter any unexpected issues or performance problems, it will be more challenging to figure out which change might have introduced the issue.
if you want to upgrade your sharded clusters while maintaining availability, you will have to plan upgrades between successive major releases: 3.6 => 4.0, 4.0 => 4.2, 4.2 => 4.4, 4.4 => 5.0, 5.0 => 6.0. The most straightforward upgrade path would be using automation tooling such as MongoDB Ops Manager or MongoDB Cloud Manager.
The relevant documentation to review is:
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Release Notes and upgrade procedures for the relevant versions of MongoDB server in your upgrade plan.
Regards,
Stennie