If a user already has a reverse zone set up, the way I see it, you don't need to ask for up to 3 servers - one should do. When the one is input (or changed), fetch the NS records from the zone on the specified server and populate your delegation records from there. This also confirms that the zone parsed and is loaded on at least the name server that was specified. If you want to test the others obtained from the NS records to make certain they all respond (and maybe even have the same SOA serial, etc.), I leave open for you to decide on how far you want to implement the suggestion.
I make the suggestion because some of us (well, I do) have more than 3 name servers that serve our zones.
