its apparantly quite easy to file an extra slot into the old style 2 speed left shifter to create an index for the 3rd gear
I never saw any evidence of this getting done successfully in practice. From my my own inspection an attempt to file an extra slot has no chance of accomplishing anything and will just irreversibly damage the shifter. 3-speed shifters can be had for cheap and there is an advantage in being able to go back to the previous configuration, if the new one fails to work out. There is an advantage in going with an indexed shifter in that the pusher should be set so that it is not rubbing the tensioner wheel in either of gear positions and thus refrains from damaging that wheel in the long term. This is trickier for 3 speeds than 2.