Well, there is a unit at Monash University (hopefully someone will know about melbourne uni too

) called "Mathematics for engineering" and is compulsory. The only thing is, you have to have undertaken Spesh with a reccommded but not inforced B+ average.
If you haven't, then you have to undertake Foundation Maths in semester 1
before you can take take "Mathematics for engineering". If you haven't done VCE Chem, then you would take Foundation Chemistry, but I don't think it stops you from any other units in first year hm hm. So looking at uni pathways, Spesh would defs be better

As for year 12, you can always do 7 subjects like ppl have recommended

But, if you feel it will be detrimental then don't do it! Apart from the uni unit (subject) selection stuff, doing chem and physics and spesh and lote works to your advantage in the middle banding process. This is a process where doing those subjects give a small "bonus" to your atar when applying for university, but there is a limit to the "bonus" to your aggregate so I would check the VTAC guide first

GOOD LUCK WITH IT

~~ PS: My mate did 4 Maths in VCE. 3 Maths isn't that bad
