Shimano Tourney (SL-RS36) 3 speed indexing wrong

I'm guessing your cable is immensely too long, and its only by chance that numbers 2 and 3 line up with a gear.

Try putting it on the smallest front chainring, then back off the cable pinch bolt.

Set the handgrip shifter to 1 and pull all the cable slack through, and do up the pinch bolt.

Some tweaking may be required.

I'm not exactly sure how that Revoshift works and I will likely never get a chance to get my hands on such a low-end thing to take it apart.

The trigger-type shifters with a needle display have an internal spring which forces the indicator needle to go to the 1 even if the cable tension is completely slack. If that spring breaks, then they will not go to 1.

Perhaps your barrel shifter works similarly.

It is not unusual for the cable tension to be completely slack on the lowest front ring. That is simply because the index position is farther than necessary; the actual movement of the derailleur is trimmed with the "L" screw. You can see this clearly in the assymetry of the typical shifter display, which looks a little like this:

3   2       1
      ^^^^^^  extra distance between 2 and 1

So that is to say, when you shift to "1", the cable becomes quite slack. The front derailleur wants to move, under its own spring tension, much farther than necessary, and will happily dislocate the chain off the small ring: what stops the derailleur from moving too far inward isn't the shifter, but the position of the "L" (low) trim screw on the derailleur itself. Since the derailleur is stopped by the "L" screw, it is not able to maintain tension on the cable, and the cable goes slack.

3   2    |  1
         |
         ^ actual position of gear "1", the point where the derailleur
           stops at the "L" limit screw and the cable loses tension.

3   2       1
            |
            ^ needle moves here anyway, because of a spring
              inside the shifter, creating the illusion of a perfect
              index position.

If that shifter's internal spring were to break or dislocate, then the shifter would rely purely on cable tension, and will not indicate 1 reliably. The display will move only as far as it is pulled by the derailleur, until that derailleur encounters the "L" screw. At that point, the cable goes slack, and so the indicator is somewhere between the 1 and 2. No shifter spring, no illusion!

Perhaps the Revoshift barrel shifter is also spring-loaded in a similar way: some internal spring that is supposed to rotate the dial to show 1 has stopped working.

Guys thanks for all the tips. I took apart the switch and put as it is necessary. Now switch works as it is necessary. It remains to hang him on a bicycle. Yes, in this version of the switch all the gear and guides. Thank you! P.S. This is my first bike, the best budget.