Just had a look at the possibilities, assuming you aren't doing specialist maths.
It's pretty flexible, but you would not be able to fit a triple major in (ie maths + statistics + bio-related).
See attached BSc course map I put together for reference below. Managed to get a Double major in maths & stats, a minor in chemistry and a minor in biological or medical science (several options). Click on any of the unit codes and it will take you to the Monash handbook entry for that subject.
- Stuff highlighted in yellow is the double major sequence in Mathematics & Mathematical Statistics
- Stuff highlighted in blue is the chemistry minor
- Stuff highlighted in green is the additional bio/med sci minor options: the 1a+1b sequence is molecular biology, 2a+2b is biochemistry, 3a+3b is developmental biology and 4a+4b is microbiology. There are others to choose from, but these seemed most relevent.
There's lot's of ways it could be put together, this is just one. I'd recommend taking the course plan to the science student services office and get one of the advisors to go over the option with you.