I'm hoping someone can help me...

I have tried (unsuccessfully) to change the light switches in my lounge and upstairs landing. Here's the scenario.

Lounge has a Dimmer Switch by the front door which controls the Lounge light.

Also in the lounge is a Two Gang switch on which one switch controls the Lounge light and the other controls the Upstairs Landing light.

Upstairs Landing - One switch which controls the upstairs landing light.

I have changed the Dimmer Switch and the Two Gang and some weird things are happening. The dimmer switch now turns on both Lounge and Upstairs Landing lights. I have also changed the two gang and this now does weird things too, like turning on all lights with one switch and only upstairs with the other. I have not changed the upstairs one yet.

The new fittings I have have the following terminals

L1
L2
L3

The wires I have are

RED
YELLOW
BLUE
(EARTH which is connected to the back box)

Can someone help me on this... it's really fustrating ;-(((

Thanks

Don't touch the upstairs yet, Can you put back how it was, Some info is lacking.
Red, Yellow and blue, is this on dimmer/switch combo? Or the wires in switch box?
A 2 gang box will handle 2 devices. A stacked or combo device controls 2 things in a single device.

I tried to put it back to how it was but no joy... I did write down what came from where but must have done it wrong ;-(

OK.. the dimmer switch has 1 RED, 1 YELLOW and 1 BLUE (which I have connected and it worked fine before I mucked up the 2way switch across the lounge). The 2 way switch has 2 RED, 2 YELLOW and 2 BLUE. The cables come into the box with a grey sheath and white sheath. The dimmer switch wires are in a white sheath so I presume that the downstairs lights are in the white sheath and the upstair light is the grey sheath...

Where are you located. Your terminology makes me think that you are in Europe or at least not US. What voltage system are you on 120V or 240V. To us in the US, a 2-gang is just a box that will accept two devices or switches. A wiring arrangement that allows a light to be controlled from 2 different places is referred to as a 3-way switch (I know that doesn't make much sense but that is the way it is).