A three-gang switch with two one-way, and one two-way (stairs). The trouble is, there are hundreds of ways of wiring this arrangement to the switch. Every electrician has his own way (to a point). One of the three wires for the stairs will be live. Often, that has a link (short piece of red or brown wire) over to the other two one-way gangs. Perhaps that's come off.
It's usually fairly easy to trace, if you're experienced. (Don't let anyone tell you you're not allowed, by law, to do this ........ you are!)
Simple substitution of the switch is probably the best you can do before you have to call someone out.