Re: Commerce/science, commerce/arts or single commerce at USYD? Which is most benefic
As has been said, the Bachelor of Commerce (Liberal Studies) program would suit your needs quite well, however, I believe there is one major drawback. To qualify for the degree, you need a minimum of 108 credit points (18 subjects all up) from the Business School. Obviously, if you were doing 2 majors, you would have plenty of space free to fill up your units with elective business units or extra units offered by the department that you are majoring in. But with 3 majors, especially when only one of them is a business major, you may struggle to meet these requirements. Of course it can be done if you plan well, but in the space of 4 years, I think you'll find if you manage to complete the 3 majors that you want, you may not have been able to study your area of interest as deep as you may have wanted (unless you place more emphasis on getting out of uni as soon as possible to get a job then acquiring knowledge, in which case Liberal Studies would be good).
An alternative that would give you more flexibility would be Commerce/Science with a concurrent Diploma of Language Studies. It's five years long, but I think the extra year is worth it for the flexibility and extra study options it gives you. For the Diploma, USYD are offering an intensive pathway starting from next year where you can complete the diploma in one year without it disrupting your studies. This will only be offered for Spanish, Japanese, Indonesian and Korean, but it will allow you to study a language without having to forfeit any units in your study program each semester to study it. Or you can do the Diploma part time and do one unit per semester on top of your existing workload (this would mean you have a choice of more languages and can study it in more depth, but the trade off is that you'd be doing more than the standard semester workload, so only do this if you feel confident you can handle it). I'll actually be doing the latter option starting from next year.
In terms of your concerns of employability, having extra knowledge can't do anything but help your employment prospects. Again I think the trade off is that if you do straight Commerce you would be out there in the workforce earlier. But otherwise, if you have a genuine interest in areas other than Commerce, I would advise you not to worry too much about employability and do what you enjoy, it will pay off for you in the long run. As for USYD v UNSW v UTS, they are all great business schools, so really just go to the Open Days and decide what uni you feel like you would be more comfortable in. I can't speak for UNSW or UTS, but I can personally say USYD is fantastic for the all the areas you want to study in, which is part of the reason why I decided to go there.