It's generally a good idea having at least one ally following you the entire time, since you won't know when you'll need him/her. When playing alone or with one friend, the teamwork sections would always necessitate two players to cooperate. If you don't have a single ally by the time you reach said section, you could always backtrack to find a foe to befriend.
Thanks for the answer! I was just curious because it seems as though the game is too easy when you have all 3 allies. Maybe when I do end up getting it I’ll jusg try to keep one around until more are absolutely necessary