Changing switch to dimmer (UK)
You have three wires because its a two way (or more) switch; You will find your light is switchable in two (or more) locations.

The wavy line with an arrow is the 'load' terminal, meaning this should be connected to the light; either directly or though the other switch(es)



It is likely that the old L1 goes to the load; the other two won't matter.

To be sure you need to test that the wire does indeed go to the light; this will require some electrical test equipment, and may require testing live circuits, so shouldn't be done with some experience.