To get all three legendary birds you have to have Charizard as the only person in your team and he should be holding a Friend Bow. Then go to Mt.Blaze,Frosty Forest,and Mt.Thunder.Best if Charizard is lv.100.Anyway,beat all three legendary birds with the attack Overheat. Then they will join you.For Lugia,try to bring a lv.100 Swampert with a Friend Bow in Silver Trench.If you beat Lugia,he might join you.As for Groudon,do the same with the lv.100 Charizard holding the Friend bow.