Absolutely. Pretty sure all experience is party-wide, at least it’s that way in the single player. Every character in my party has identical XP.
So XP rewards don’t scale based on number of party members, since players (human or AI) can drop in and out at any time, and will inherit the party’s current amount of XP.
You still may need a fourth character as I’m guessing the game’s difficulty is tuned for four. Or just turn down to Explorer at the start until you get some gear.