Ok, make sure the script I told you to change says "3" instead of "4". The only party changing script that I know of that will work is Blizzard's. Check the script database for it. Let me check to see what line you have to change to keep the player from adding more than three.
EDIT: Ok, right after the instruction crap is this bit of script:
#::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
# START Conficuration
#::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
# how many party members do you use
MAX_PARTY = 4
# set to true to use facesets instead of spritesets
FACESETS = true
#::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
# END Conficuration
#::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
Change "MAX_PARTY = 4" to "MAX_PARTY = 3"
That should solve all your problems.