1. A Commerce degree is 144 units of credit, I think. You do 48 units (8 courses) 'core', and then there are up to 42 units of credit each per major. So with two majors you only have as little as 12 units of credit to go... so clearly, you run out of space and if you want to do 3 majors, you're going to have to take extra time for your degree and I'm not even sure if that's possible, go ask the ASB.
2. You take 4 courses per semester, normally. The workload for Commerce is on the light side. You cover an entire textbook in 13 weeks pretty much - per Commerce course. Is it a heavy workload? It's as heavy as you want it to be - because no one is going to care if you do your work or not.