Double light switch

[AnonymousImport]

In a 1998 built house I have 4 ceiling lights, L1 - L4, and 3 light switches S1 - S3. S1 & S2 are standard single gang switches. S3 is a double switch with 4 x 2core and earth wired to it. I don't know if the wiring of S3 has been altered since installation.
Each light is wired by 2 core and earth cable. L1 & L2 are independently operated on/off by S1 & S2. S3 operates L3 on/off, but if S3 is on, then S1 & S2 do not work. S3 should operate L4, but L4 does not work with any switch. Any suggestions please?

 

[AttM]

How many cables do you have in S1 & S2?. If you have 2x cables in each then it sounds like the power loop has been run through the switches, unusual but not unheard of.
This means that you will need to test for a complete power supply (live - neutral 230V) in each switch with the other switches being operated and see if you loose the voltage.
This may, or may not, give results but will point you to the next step.