Bet there is a dial or table with the same three colours on the back or side of the flash.

If not - see the bottom of this post.

Pick a colour, and set the switch to that colour. Then look at the dial or table, and you will see numbers representing distances, film speed and lens apertures.

If you film is ISO 125, there will be a correlation between that, the colour you chose and an aperture. Set your lens to that aperture, set your camera to its synch speed, and providing your subject is neither two close, or too far, the flash will measure light bounced back from your subject and adjust itself accordingly.

Usually you can see a correlation between the colour and a range of distances.

One exception - I've actually seen a couple of flashes that are so cheap that they just re-used parts from more expensive models. As a result, they had multi-colour swithches that did nothing.