mongo shard balancer not working
First, restart the mongos process on mongosHost3, then recheck the locks as you did in the question.

If the same lock (make sure it is the same and not a new lock by the same process - check the ts field etc.) is still present after that restart it is safe to remove it manually, it is basically stale and should be cleaned up eventually in any case. You can remove it just like any other document (assuming you have permissions to edit the config DB of course). MongoDB 3.2 introduced the deleteOne command which I would recommend using here.

It is possible that this is not an issue with a stale lock and is instead an issue with balancing (i.e. it is seeing errors or is stuck/slow for some reason), take a look in the mongos logs of whatever process holds the lock once balancing kicks off again and see what it reports as progress for balancing.

You can also look in the the changelog for more information related to balancing and migrations. If balancing is seeing issues I would recommend getting the logs, changelog information and posting a separate question to troubleshoot that issue.