What Shard is, not very politely, saying is which game are you talking about? Since you're asking to go from three to either one or two party members I can assume you're talking about either FF7 or FF8. You just need to be more specific on which game and platform you want such a tool for.
EDIT:
For the FFVII PC (Even if that's not what you want I need to document it) you just have to use a hex editor to change the value at 0x300081 from 74h to EBh. That'll trick the PHS into thinking you have a full party. Still have to have your lead character in the party, though.