Your plan for three Australorp at 5 weeks old will work, if you can find them. It would be especially nice for you since they were developed in Australia from Orpington stock. That may be a challenge getting only three and getting them the right age. If you run into problems with that, you might want to look into this link. It is the Australian thread in the "Where am I" section on here. You may find someone close to Perth that could help you in getting young pullets, either from their flock or information on where to find them in Australia.
https://www.backyardchickens.com/forum/viewtopic.php?id=186559
If you are unable to find Australorps and have to order from a hatchery, might I suggest you look into the sex links. They are called sex links because of certain sex linked genes passed only from the mother to the son. The daughter does not get these genes. Certain of these sex linked genes are dominant over other certain genes, such as silver over gold or barred over not barred. You can tell very clearly at hatch if a chick is male or female if specific parents are used. Different hatcheries call them different things like Cinnamon Queen or Golden Comet. Star is used in a lot of the names. These are not pure breeds but are crosses. They are normally excellent egg layers and pretty docile and friendly. I think they would be a good choice for you and your circumstances, both with no roosters allowed and with children.
I also think that Australorp, Orpington, Delaware, Sussex, any of the Rocks, Easter Eggers, Brahma's, or Dominiques would be excellent choices if those are available as older, sexable chicks from a neighbor with a flock. And in your case, there is nothing wrong with mutts, chickens of mixed heritage, especially if the above are the breeds mixed.
If you are in the position of picking from an existing flock or if you order sex links from a hatchery, you might consider mixing the chicks so you get different colors. With only three hens of one breed there will probably be enough difference in size and conformation that you will be able to tell one hen from another pretty easily, but you might enjoy the different colors of a mixed flock. Often with a mixed flock, you can tell which hen laid which egg by shape or color. That is not only neat and cool, but if you have one that lays a deformed egg or has some problem with her egg laying, you can often tell which hen laid it.
As a final suggestion, you might consider setting up your profile (in the blue bar above) to show your location. Not only might it help you find a neighbor interested in chickens, it does help in some questions and comments on here. For example, since you are in a relatively mild climate, you do not have to wory about getting an especially cold-hardy breed. That really opens up the choice of breeds to recommend.
Good luck on the adventure.
