Littletee, have a navigate of this:
http://www.dse.vic.gov.au/planningschemes/brimbank/home.html
That is the planning scheme for Brimbank.
I was under the impression that you had approval already for three. Ideally 900 sqm or greater would be the go, however depending on the council you can go denser. You also have to be mindful of what footprint they will allow you to place on the site, open space, shadowing, orientation, privacy etc, etc.
Hume council for example is pro-development and three would get on that site however not three at 3 BR each. Probably One 3 BR and two 2 BR's or some slight variation.
As Christian mentions and I also outlined in an earlier post, it depends upon where your site is and what council would like to see there. The closer to a transport or shopping/services hub it is the more likely you can go denser than they would normally encourage.
Make an appointment with the council town planner and have a list of questions at the ready, before you engage private consultants that you will pay for.
Have a look at the site on the link above.