3 degrees is -16C. Ellie regularly starts usually on the first crank of the starter in even colder weather. I do put her on a battery maintainer. Not because I need to but because it just makes good sense.
Temperature and voltage of an electric system is not as related as folks think. Starting amps and temp is related though. For Cold Cranking the CCA number must be much higher. If you use a battery maintainer then the CCA number need not be as high.
I grew up in Northern Ontario with temps down to -60F in the late 50's, 60's and early 70's. Lived in Inuvik for 3 years in the late 70's with temps down to -60F. Spent 2 tours in Alert -- look up Alert on a map sometimes -- -60 with windchills much colder. Vehicles that were kept sheltered didn't need special treatment. Equipment kept outdoors better have had a minder on them or had block and battery heaters, circulation heaters and high CCA batteries. Sometimes they would still need to be warmed up under a tarp with torpedo heaters. Lived in Dieppe 26 years now, winter temps average for weeks in the -30's c and Ellie starts whenever I ask her to pretty much. Only time she doesn't is when she was left outside for extended periods.
If you Cub is sheltered, you electrical system is in top shape - meaning all connections clean and bright, no cracks in the insulation etc., and the battery fully charged it shouldn't matter what voltage you have period.
12v or 6v - same diff - both need the proper battery for the conditions, shelter and a maintainer to keep them up to snuff. Unlike cars and trucks our Cubs don't usually run long enough to fully charge the battery when the lights are being used or even if you are just plowing the driveway.
Yust me dos centavos worth