Three different people from the same company come in and charge lunch to the account. Is that allowed?

Yes. A house account belongs to the company, not to one person, and any of them can charge to it as long as your cashier picks the account on the Charge screen. Every sale lands on the account's statement with its order number and the time, so you can see who came in when. Set a Limit on the account under Settings → House accounts and the POS refuses the moment a sale would take the company past it, whoever is standing at the counter. Zero means no limit. Keep Contact and Phone filled in so you know who to call when the month's bill is ready.