Your switch may not be exactly the same as the one in our '84 Spider but here's a look inside the switch in our car: Combination Switch. Maybe you can use that info to get inside your switch and figure out the problem?
Your question is a little confusing, as the turn signal / light switch on a '75 Spider has two levers - not three. One lever controls the parking & headlights, while the other is for the turn signals. What's that third handle on your switch?Does any other Alfa model share the same turn signal column switch part (with the 3 handles) with the '75 Spider?
I wouldn't call it "simple", but see the excellent write-up at: http://www.alfabb.com/bb/forums/spider-1966-up/429249-inside-combination-switch-exterior-light.htmlis there some simple fix for a 'floppy' set of controls