Whatever happened to Radaghast?

[Olmer]

No, Valandilthis time it's you who suspecting a foul play. Bad examples are catching.
I suspect nothing of such kind. Actually, I think that Radagast-Aiwendil has been living happily ever after in the woods of Middle Earth, talking to birds and animals, and waiting for his time to come, when he will play a major role in the enthronement of another imposter, who would be a great king.
People called him Myrddin -the shapeshifter in the woods. Later it was changed on Merlin. Guess, who was that mudblood placed on the throne with his help? The clue - his name also starts with "A"

I am sure that other two istari did something similar in the others far away countries. After all they all saw the Music of Ainur, saw the future and knew what each and everyone has have to do in farther development of ME's history.