Number 1 is mixture, very sensitive. A eighth of a turn (on both carbs) added 10bhp+ to my engine when being tuned on a hub dynamometer. Number 2 is idle speed. Number 3 is fast idle triggering point.
If you move the choke lever on the carb, you can see the bottom of it move closer to the fast idle screw. You can also see how it pulls the jet down. The mixture screw bears on the choke lever and sets the lever's upward limit, and therefore the jet height and mixture once the choke has been dispensed with.