according to the connect!on interview summary Reinheart Translated you get the same benefit if you have all 3 companys in a party as you get when all of the party are from the same.

When everyone from same company you get bonus as well as when you have 3 different countries grand company members
heres Reinhearts full post: http://forum.square-enix.com/ffxiv/t...l=1#post356556