You would have to go to Guest Relations in the parks to see if they would transfer the fast passes. Fast passes and magic bands are non-transferable. Technically, in order for a fast pass to be valid, the person would need to have a regular theme park ticket, and have scanned their magic band\ticket for admission. You definitely cannot use the under three magic bands to use fast passes. Under three, as you already know, don't require them or tickets. What the cm did, he "accidentally" added the fast passes to three under the age of three.If
You were gifted three fast passes for those who were eligible, not for under three. The cm just didn't look close enough. I am going to assume this was handled over the phone and not at the front desk of a resort, or in the park. When you call you are talking to "cast members" which is actually a call center.
Remember it doesn't hurt to ask!