Also, why a pair in series and four pairs in parallel? I would only want 8 actually, but why that configuration, instead the 8 in series as suggested earlier?
8 in series required high voltage : V = N x 3 V = 24 V, or even more. I'd suggest you connect all of them in parallel, and run from on-board +5V with small resistors in series with each laser to drop 0.5V as it was suggested. You wouldn't be able to get stable brightness connecting two units in series, though it may be o'k with switching power supply (price ^). With battery running down, or cheap wall-wart power supplies you would get a lot of troubles to keep current more or less constant.