More helpfully - how old is the 3 year old? Just 3 or nearly 4?
I would watch Shae by Air (available on Amazon) if they are quite young or not used to flying. It's a DVD just for children about flying, airports and how to behave (ie no kicking the seat in front etc).
Does your airline have a children's channel in seat back screens? Still worth taking a portable DVD player if you can - this is always a lifesaver for me on flights.
If they are nearly 4 a leapster is good.
If you get the bulkhead seats (more room and nobody in front to kick) bear in mind that the arms don't fold up, so they can't lie across you to sleep (my BIG mistake).
Take some new little toys - magic colouring books are good (the ones that come in a foil packet with pens that only mark the special paper). Invisible answer books also good, they do them for littlies. Wrap them and bring them out when boredom strikes.
Take lots of food so you aren't at the mercy of the airline (you will have to ditch this when you arrive in Australia).
If you are thinking along the Phenergan lines, test it out a week beforehand, some it makes sleep, others it sends hyper.
Are you taking night flights or a mixture? I flew on two night flights with a Hong Kong stopover. These are better if you know they'll sleep, but also more scary if you think they won't sleep and you'll have to keep them quiet for 12 hours while the rest of the plane sleeps (or not in my case, as the screams travelled quite a long way).
I'm sure it will all be fine . Are you flying to Sydney?