Yep 6 minimum, with a 3ft tank Im guessing we are talking 30/40g? So with 6 congos, reaching full size of 5" thats 30" of stocking, leaving you IF the tank is 40g with 10" of stocking space (using the 1" per G rule, which isnt always steadfast but a rough guide)
You dont want to put any other type of tetra in there as they will probably nip the congos long fins and as said no tiger bars or other similar nippy fish.
Congos can be very skittish fish, they like plenty of planting and can be a little sensitive to bright lights so its a balace, you could use some floating plants to lessen the lights if they are too much for them.
I would suggest (and its just because I like cichlids) that you could add a pair of rams, blue or bolivian or perhaps keyhole cichlids as they will not bother the congos nor the congos bother them, providing of course its a 40g tank, not 30g. Although after a period of maturity for the tank (6 months minimum) if you water stats are spot on, ie: no ammonia, nitrIte and low nitrAte levels (20-30ppm) you could try for the cichlids mentioned.