you should be able to do this in mame.
Start the simpsons, hit tab, go to "controls for this game" section (input/this game only or whatever). Go through Player 3 joystick and buttons and assign them to your P1 controls.
However, you will need to keep the P3 coin-up and P3 start buttons seperate - so you could simply map those to the shifted functioned on the ipac for the P1 coin-up and P1 start buttons.
Then, to play as p3 you simply hold your shift button, use P1's coinup and start, release the shift and you are good to go.