Dwarf puffers need 5 gallons per puffer, I tried to keep 3 in a 10 gallon and the smallest was killed, you need atleast a 15 gallon for three and even still it must be densly planted to prevent the DP's seeing eachother too often. In your tank I would not suggest having any other fish, however, if you feel you must perhaps a trio of oto's or a pair of Khuli loaches would be OK. DP's IME seem to attack any midwater to topwater fish or with flowing fins so the choice for tankmates is limited. If you do decide on more fish, make sure you have regular maintenance on the tank, even DP's on their own produce a lot of waste.

Cheers,
Mike