For overnights, I have the large expandable brief from Briggs & Riley. Technically, it can do 3 days of business casual but I'd still need another bag for shoes.
So for the 3 day trip, I'd either use my timbuk2 aviator or my T. Anthony canvas suitcase because I can pack my briefcase into it.
Longer than that I use a rimowa salsa air which I can live indefinitely out of, plus a brief.