Yep, three. I think if I really, really wanted to, I could probably get four jammed into the lower part, but it would involve shaving the rubber on the kegs down, and I'm not inclined to try it.
Here's a pic of three kegs in the lower part:
I'll also modify my earlier statement that two kegs will fit on the hump with a collar installed: They will, if you don't have taps over there. My four taps are all over on that left side, so only one keg will fit there.