The Young One - your opening post asks how many male bettas you could fit in a 3 foot tank. Do you mean divided?
If divided - as long as you give them 3 gallons of water each as an absolute minimum, and as long as you take into account the amount of space taken up by decor and substrate, then you're good to go. With a tank that size, I'd be aiming for 4-5 gallons each. You could still get a good number in that size.
(I'm talking UK gallons, btw, so that's about 4.5 litres per gallon)