Every original bike I've seen had one. I always use three. I detest the jumble of wiring and connections under the tank so when I wire a Norton, it doesn't exist and most connections are made in the headlight shell. Three holes are easier for this. To see what I mean, look here:
http://www.gregmarsh.com/MC/Norton/Norton_1974_Wiring.aspx
Of course, a purist will hate how I do it but I have less connections to fail and no need to remove the tank if one does fail.