This is actually a pretty interesting topic. I was just moaning in guild chat about how I'd really like a third profession (and quad-spec, but hey).
I really love Alchemy and Herbalism, but I don't find that I make a lot of money off of it. I'm considering switching Alchemy to Skinning. I'll lose out on so much, like gem transmutes, endless potions and cheap brews, but on the other hand I'll probably make a killing from selling leather and herbs instead of consuming them, heh.
I've also been wondering if I should pick up Enchanting, because I run a lot of instances solo and notice that the disenchant value on drops is about 10x the vendor price. With the new DE option in 3.3 or whenever, it might not be an issue, though.
God, I wish I could pick up every profession. I don't play my other characters so much and I can only really see my Druid as my gatherer thanks to Flight form and stuff.
____________________________
Please "talk up" if your comprehension white-shifts. I will use simple-happy language-words to help you understand.