Well you know blizzard and their attention to detail. I think for the expansions they're going to add units and other tidbits along with the campaign. They've been pretty tight lipped about everything. I mean diablo 3 is in the same situation and it isn't getting broken up and still going to take who knows how long.
Also, I didn't mean 3 years for the expansion but rather 3 years if they hadn't split the game into 3 pieces. So the overall product would have taken 3 more years, but since they're doing it in pieces I'm guessing it'll take them 3 years to release the last expansion.
Blizzard's in good business though, the game brought in at least $90 million on just the first two days, who knows how much more since after that.