Don't do Liberal Studies, it is extremely over-hyped. You can cover the content much easier and in much more depth with a combined Commerce/Arts or Commerce/Science degree.
Cramming three majors into a single degree might prove a little bit difficult, so the way I see it you have two choices:
1. Do Commerce/Arts, with your two commerce majors and an economics/econometrics major for arts
2. Do a single Commerce degree, however, don't plan to do one of the units that is compulsory for your degree until the very last minute. The university will force you to graduate when you have completed all the requirements of your degree (they won't allow you to go over), so if you wish to stay in university until you've finished your third major, I would put off doing BUSS3500 until the very last minute, preventing the university from forcing your graduation.
I'd recommend the first option though. Saves you having to explain yourself and encountering possible administration issues. I'm doing the second option myself, but from my experiences the Science faculty at USYD is a lot more flexible than the Business faculty.
As for the difference between economics and econometrics, econometrics is a more specific area of economics that involves using a lot of mathematical models and statistics to describe economic systems. Economics itself is more general.
