3 now belongs to Vodafone.
Virgin is part of Optus
Telstra offers the best and widest coverage but is horrendously expensive for calls and data.

Optus and Vodafone have a good non-contract monthly call data plans offering unlimited local calls and SMS messages and 2Gb or 3Gb of data for about $110 (60 pounds) per month.
They obviously have cheaper calls too.