I don't think you should keep red-tailed sharks in a tank that size. What waterlogged says is true, but the sharks can get quite large, so would do best in a 200L-250L tank or something around that size.
How high and wide is your 3ft tank? Depending on what species you choose, you could keep a couple or maybe only one. Dwarfs are a bit aggressive, mainly when spawning. I would go for a pair of Apistos, and maybe a couple of pairs of Blue Rams (Microgeophagus ramirezi). I highly reccommend Apistogramma macmasteri, because they are very active and have a great personality, and have such beautiful colours! Hopefully with lots of people trying to breed them they should be availible soon.
Dwarf Cichlids do best in planted aquariums. If you just want a low-tech tank, you can use plants like Ambulia, Cryptocoryne sp. (like affinis and willisii), tiger lotus, rotala etc. You can use DIY CO2. Substrate can just be gravel with JBL root balls under the root-feeding plants, which is the crypts and tiger lotus. And for ferts you could just use PMDD or Flourish Comprehensive. For lighting 4x30watt T8 tubes will be sufficient.
Other fish that you can keep with them happily are small schooling fish like tetras, and you could also have some larger surface-dwelling fish like gouramis. I think a school of 15 Cardinal Tetras (Paracheirodon axelrodi) and 9 Kitty Tetras (Hyphessobrycon heliacus) will look splendid in a low-tech planted tank with the dwarf cichlids.