15 gallons is not much of a tank to work with, espicially if you already have 3 mollies in it. I wouldn't reccomend adding any more fish to the tank unless they are balloon mollies, which grow considerably smaller than the sailfins and shortfin mollys, which in some cases grow as long as 6". Plus, the requirements of mollies in regards to the water chemistry differ from the requirements of the neon tetras. To thrive, neon tetras need soft acidic water with a
pH of anywhere between 6.0-7.0, whereas the mollies need hard and preferably brackish water, with a pH over 7.0