The Amigo Outfit is composed of the Amigo Hat, the Amigo Jacket, and the Amigo Pants. These are most likely a reference to the film The Three Amigos. They are apparel items not armor items and are visibly equipped on the party.
If three of your party members are wearing the full suit, the noun "Rangers" may be replaced with "Amigos" in some dialogue. An NPC may, for example, say "Thanks for the help Amigos" rather than "Thanks for the help Rangers."