Replacing 2 gang with 3 gang switch

[Div]

On my 2 gang switch, one (one way) switch controls both the hall light and the external light. (the other 2 way switch controls the landing light). I want to separate the hall light and external light to two separate switches on my new 3 gang switch plate. How should new 3 gang switch be wired up?

 

[ban-all-sheds]

Like the existing one, except that the live and switched lives for the hall and external lights should go to separate COM and L1 terminals, not the same one.

That's aways assuming that the switch is wired that way, i.e. the switch cables for both lights into the same switch. Have you looked?

If instead the external light is wired to the ceiling rose of the hall light then you've got a completely different job to do.

 

[Div]

Thanks, I haven't looked yet but expect to find live and switched live to both lights in the same switch. If I mess up in my 3 gang switch such that the live from one light is matched to the switch live of the other light (and vice versa) will this be dangerous - if so I think I need to get an electrician to check the wiring!

 

[ban-all-sheds]

...If I mess up in my 3 gang switch such that the live from one light is matched to the switch live of the other light (and vice versa) will this be dangerous...

No - if you think about it the live feeds (if your expectation is correct) are already connected together, as are the switched lives. It would be better to separate them consistently with the new switch to make life simpler for any future changes, but if you can't tell which ones are which it doesn't matter.