BTW Up+Down is a ok solution I think. Most joysticks can be easily modified to use either of their buttons to work as this combination.
for amiga joystick this mod is really easy, you need just three wires and two way switch. ive used "reset" switch from old pc case.
With third button it will be really problem, as none of amiga joystick dont have enough wires in cable. But there are solutions which i have in mind:
-Butchering cable from sega pad (enough wires), or
-Using sega pad directly for moddding (i dont like these, really).
-Make a completely new cable (does not look good for me)