Downstairs I have a ‘dumb’ triple light switch (like this) which controls the outdoor porch light, the downstairs hallway light, and the upstairs hallway light. The upstairs hallway light is in a 2 way configuration, with the second switch being a single gang switch upstairs.
So only 1 gang on the switch is on a 2 way wiring? Just to clarify
Yes, you will be reusing the existing 2 way wiring.
If you don’t intend to add the extra live wire, then you’ll need to change your 3 gang switch to a momentary type or convert one of them to momentary
https://preview.redd.it/r2bohuyj6t0c1.png?width=1280&format=png&auto=webp&s=2afa82ca26a0b3f8599441c5a6c02bafc931f308