as it's an AC switch, sounds like you've already used all the options.
all position l does is disconnect the CDI from earth, nothing else is switched.
but do you need a seperate sidelights and then headlamp positions ?
you could use position ll to switch the power to the gauges.
so [assuming it has colours as per the ones from Scooter Restorations], have the blue from the ignition plugged into a live power, this connects to the black & yellow in position ll, so use these for your gauges.
in position lll brown is conected to orange, so use this to feed your lights, inc to take power to the rear lamp via the [usually] black wire, speedo, festoon and to the main/dip switch.
what handlebar switch have you got ?
then the guages should work in pos ll and continue to work in pos lll when the lights come on too. they won't work [but the engine will run] in pos l
hth
edit: I missed off a "l" earlier
, blue connects to black & yellow in position ll