Hi.
Debra has given you accurate information re the three 'entries' that appear at the online INDEX. I think that the explanation you have been given over the phone from NSW BDM is actually from their call centre team, and of course, the call centre does not have access to the actual record and likely have not ever been working with the actual records, so umm... may not understand why there's often several 'entries' in the index for the one event.
May I suggest that you consider obtaining an official transcription of the actual marriage registration from an Official Transcriber, appointed by NSW BDM.
ADD ... likely the column where the bride's surname is recorded has more than one surname listed there. So the index has more than one entry for the one event.
JM