It really depends a lot on the exact context, as to whether or not 'sector' is the right word — of course, we're not talking here in the wider sense, like 'public / private sector', but a 'sector' of the kind of business they carry on. But of course, this word may not be ideal for certain types of business, and I would try and find the word that is most pertinent for the particular trade they're in.

I don't suppose it is the case, but one must just acknowledge the fact that 'pôle d'activité' could also mean an actual, physical site on which they operate — but I feel that is unlikely here!

or "have", if there are many companies. You could just drop the "structure".