You should note and check what each wire does rather than rely on long distance advice over the Net.
By the sound of it, it is likely that one of the reds is the incoming live mains. This can be checked with a neon screwdriver if you are careful and don't touch the live wires, or accidentally touch them to something else. One is likely to be taking the mains to something else that needs it. The last wire, and I'd hazard a guess it is the one on it's own, is likely to go off to the lamp to light it.
You need the switch instructions to work out the connections there. I'd suspect the common is probably the one marked F but in all honestly I'd be loathe to proceed without the switch instructions. I'd hate to suggest something and you then blow something up because of it.