当做, 当作 (dàngzuò), and 当成 (dàngchéng) are interchangeable — all mean 'to regard as.' 当做/当作 are more common in writing, 当成 is more colloquial. They almost always appear with 把: 把A当做B.
mistakes
Don't confuse 当做 (dàngzuò, to regard as) with 当 (dāng, to serve as/be). 他当老师 = He works as a teacher. 他把我当做老师 = He treats me as a teacher (even though I'm not one).
