nivsy The decision to cancel was yours. Presumably you either didn't want a refund (less cancellation fees), or the fare was not cancelable.
In general, Qantas is offering some flexibility with the issue of a voucher.

Best that can be done is split the booking, and rebook for those pax that wish to fly. Could the credit for the other two be used for a non-flight purchase ( hotel?) .

My main beef wiith Qantas has been around call centre response times, often spending 4 or more hours on hold.