3 and 6 are totally different games. 3 is for the NES(famicom) while 6 is for the SNES (Super Famicom), both 2 different games you cannot get FF III for the PSX since it does not exist for that platform, though it was being remade for the Wonderswan Color ( a japanese handheld that never came to the states ) though it was cancelled.

P.S- you can get final fantasy IV and chrono trigger on "Final Fantasy Chronicles", while you can get final fantasy V and final fantasy VI on " Final Fantasy Anthology", and both of these games are for the PSX, hope that helps .