ALL the Istari were there to resist Sauron; Gandalf was the only one who remained true to his mission.
In a pieced-together essay in Unfinished Tales called "The Istari", Tolkien said:
Indeed, of all the Istari, one only remained faithful, and he was the last-comer. For Radagast, the fourth, became enamoured of the many beasts and birds that dwelt in Middle-Earth, and forsook Elves and Men, and spent his days among the wild creatures...And Curunír'Lân, Saruman the White, fell from his high errand, and becoming proud and impatient and enamoured of power sought to have his own will by force, and to oust Sauron; but he was ensnared by that dark spirit, mightier than he.
But the last-comer was named among the Elves Mithrandir, the Grey Pilgrim, for he dwelt in no place, and gathered to himself neither wealth nor followers, but ever went to and fro in the Westlands from Gondor to Angmar, and from Lindon to Lorien, befriending all folk in times of need.
The remaining two, the Blue Wizards who we only know by their names in Valinor as Alatar and Pallando, are said to have gone into the east; but what they did there, we do not know. Tolkien said in a letter that they may have inspired "secret cults and "magic" traditions that outlasted the fall of Sauron."
It is interesting to wonder what might have happened had Saruman and Radagast remained true to their mission. Could all three Istari have travelled with the Fellowship? If they had, could they together have defeated the Balrog more easily, and been with Frodo when he had to make his final choice? With three Istari involved, might Gondor and Rohan have faced the forces of Mordor with greater strength? We will never know.