Technically you can do 3 I think, but in reality you may not even be able do two... The majority of majors (certainly the ones I looked at in any great depth: accounting, finance, economics and information systems) all require you to take extra subjects in addition to your major subjects to gain recognition with the various professional organisations out there (eg. ICAA, CPA for accounting, ACS for infosystems). Recognition is basically essential for employment in those sorts of areas. There are some exceptions... international business for example has no professional body.
Plus its further complicated by the structure of the commerce degree... to the best of my knowledge it goes:
* First take 4 subjects compulsory to all commerce students
* Followed by taking 2 from a pool of 6 subjects
* Afterwards a combination of courses to make a major sequence, leftovers becoming faculty electives.
Sometimes the first subject of a major sequence is one of the compulsory subjects... I think for accounting the first 3 courses for the major are in the compulsories even... international business on the other hand starts only after the compulsory subjects are over, eating up a whole lot of your left over subjects...
I think you can still do two majors and meet requirements for them both if you're doing straight commerce... gets even more complicated if you're doing a combined degree... trust me I spent at least an hour discussing this during the formal enrolment process and I was still confused afterwards... so its really worth contacting the university to discuss the matter further.