The ones that say "Vitamins A, C, E Fortified" or "Calcium Fortified" are somewhere between the regular and low-sodium.
The "Lemon Twist" and "Hot and Spicy" are even higher sodium than regular, so I would suggest an actual lemon or some Tabasco added instead.
I would say 3 cups is probably ok with low sodium, but I wouldn't drink that much of the regular V8 all the time. My boyfriend is a salt addict, but my feeling is that if you don't eat as much salt you get used to it, and then salty stuff tastes too strong.