Hi Fab Fiona - A 3 year old should not be paying full price on an international flight.

An infant (up to 2) is free on any domectic route and 10% on an international route.
A child is from 2 up until they turn 12 is full price on a domestic route and 75% on an international route.
Once they hit 12 they pay full adult price.

Just keep in mind that not all Qantas flights accrue FF points, generally the cheaper fares and sale fares either dont accrue them on only accrue half a point per mile. This is one of the biggest things people ask me as a travel agent and its a PITA trying to explain to them why they dont accrue them on every flight. Have a fab time in London!

xx