This is really impossible to predict because it's so dependent on the circumstances around your case and there are many variables. The court looks at these matters on a case-by-case basis, but it follows a standard set of rules outlined in the Family Law Act 1975, which stipulates that all parenting orders made by the court must reflect the best interests of the children.
The court takes into consideration the elements outlined in section 60CC of the Family Law Act 1975 when deciding what constitutes a child's best interests, so I suggest you have a read of it to get an idea of what the court would want to know:
http://www.austlii.edu.au/au/legis/cth/consol_act/fla1975114/s60cc.html
My personal view is that you might experience difficulty attaining a 50/50 arrangement for the youngest - it's generally held by psychologists that infants who are separated from their primary attachment figure for any more than a few hours tend to experience some distress and anxiety. As they grow older, of course, they transition from recognising only one primary attachment figure to recognising multiple attachment figures, and they are more comfortable with being away from the primary attachment figure for longer periods. It's usually around the age of two that kids make the move to overnight time with the non-residential parent, and then that time can continue to increase to several overnights in a row.
There's no formula for this, though. Some kids cope better than others, especially where the children have a good relationship with both parents and the parents are aligned in their commitment to ensuring the children's needs come first. Don't be too upset with the term 'primary attachment figure', either. It's a psychology term, and it must not be read as 'more important parent'. Both parents are equally important, but young kids simply rely more on one for fundamental needs when in their infant years.
Further, while it's generally preferable for the kids to share in the same care arrangement, it's also not always what's best for the kids when there's one infant and several other older children, so the older two children should be considered uniquely, rather than lumped together with the youngest. The older kids may prefer to spend longer periods of time with you, and may even resent having to spend less time with you because of their younger sibling, which may cause a rift in their sibling relationship. Similarly, they might respond poorly to spending too much time separated from the youngest sibling - again, unique circumstances of your case must be considered.
I can't advise what's best for your kids because I don't know the circumstances, but my stepdaughter just turned five and copes very well with equal care arrangements because she has such a close relationship with her dad. If you want a better idea of what generally works for kids, you could talk to Relationships Australia. They offer child consultations that are great sources for this kind of information and guidance.
