I don't think you want to be using $util.union - that will give you anyone that meets any criteria.

So if I have three team members: John, Jane, and Joe.

Say your first variable gives me John and Jane, your second variable gives me Jane and Joe, and your third variable gives me John and Joe - your result will still be all three of them, because all three of them meet at least one of the criteria.

Instead of $util.union, you might want to use $util.intersection, which returns just the people that are in each list you pass to it.  Or use $util.difference (which only allows two parameters) - this one excludes whoever is in the second value, from the first value.

If what you're trying to do is get the list of staff with the role, but exclude both of the prior submitters, it might be something like this:

var ExcludeAuthApprover = team.findTaskLastSubmitters(11);
var ExcludeWiresetupatbank = team.findTaskLastSubmitters(7);
var WireApproveratBank = team.findMembersByRole('Wire Approver at Bank');
var exclude1 = $util.difference(WireApproveratBank, ExcludeAuthApprover);
var exclude2 = $util.difference(WireApproveratBank, ExcludeWiresetupatbank);
$result=$util.intersection(exclude1, exclude2);

This is saying - get the individual who submitted step 11 and exclude them from the list of approvers - then get the individual who submitted step 7 and exclude them from the list of approvers - then take the individuals who are in both lists.

Please note that although I believe this will work, I didn't actually test it.