You do not need a second (alternate) file share witness. The only advantage it can give you is a slightly easier Restore-DatabaseAvailabilityGroup command should you need to run it. I recommend not specifying it at all.
If you have equal numbers of servers, whichever site can reach the FSW specified by DAG.WitnessServer is going to keep quorum and mount all databases. The other site will dismount all databases. It does not matter where the Active Manager / group owner is
before this happens.
In your case, you do not have equal numbers of servers. The site with 3 servers will have a quorum (3 of 5) and mount all databases. The other site will dismount all databases. If the site with 3 servers fails, the other site will not have quorum (2 of 5)
and neither site will mount databases until you use Stop and Restore-DatabaseAvailabilityGroup. The FSW is not used.
You should put the FSW in your site with more servers. This is because if you have a failed server in your primary site, you end up in the "equal numbers of servers" case.
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Marked as answer by
skipster
10 hours 33 minutes ago