Umm, you were supposed to be able to order only 2 of each model, and only 2 of them are $3 each (5v pro mini and 3.3v pro mini). So at the $3 price, you should have been able to get 4 boards. And note that you will need some way of programming the board (FTDI, etc.) that is not included in the board.
It is a good deal, though with the backorders now, I wonder how fast Sparkfun will restock, and whether you could get a similar clone from a Chinese vendor faster than from Sparkfun (of course Sparkfun does pay some amount back to the Arduino folks on these boards, and the clone vendors don't, but with this sale, I have to imagine the Arduino folks won't see much revenue from it).
I'm sure lots of people took advantage of the sale honoring internal Arduino day. In fact, I happened to be up about 1/2 hour after it began, and saw that the official Uno's, the redboards, and the pro minis were all sold out, but they had 400 or so of the 5v pro minis (the $3 item), and a couple hundred of the 3.3v version (the other $3 item). I went to bed, and a couple of hours later, Sparkfun had fixed the bug that prevented back orders, and all of the Pro Minis were sold out.