Those specific three have their own dedicated spectrum and don't overlap in Australia. There are many other MVNOs there that just resell that spectrum under a different brand name with sometimes better deals, but those three are the "upstream".
You choose 3 separate providers so that they don't have overlapping spectrum.
I've used 2 modems on the same provider, but with each modem locked to a different LTE band. This way, if the provider has one nearby tower on band 1 and another tower on band 2, you can use both of them.
(Note: If you're going to do this, make sure that you understand LTE carrier aggregation and have confirmed that the 2 bands are not being offered as a CA combination.)
Seems like it's win-win; if coverage overlaps then the connections are bonded and you get 3* 4G lines, and if you only have coverage on some of the networks that's still a usable connection.
What is the added benefit of having 3 different 4G connections? My understanding is that most of these networks would overlap.