IMO, the migration in your case might break things slightly. enabling TLS may make it bit more complex.
Best approach is to take all your custom searches, configuration items etc, into an app. Name the app with higher ASCII order and deploy into your existing deployment. And slowly remove all your old configurations one by one without any impact. After you do this, you have a consistent copy of Search config items which you can migrate
Then prepare the index clustering. 3 indexers is not a good figure. Better to have 4 or above (even numbers). Try for replication factor of 2 atleast. https://docs.splunk.com/Documentation/Splunk/7.2.6/Indexer/Clusterconfigurationoverview
Ensure you enable TLS and keys accordingly to communicate between SH tier and indexer. You need CLM also to manage indexer cluster
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