You have to do an internship prior to 3 years of anesthesia as an MD also. As a DO, I did the traditional osteopathic internship. (which is accepted by all ACGME residencies) The other (Allopathic) residents in my program did either surgical, medicine, or transitional year internships.
I heard rumors that the ASA wanted to make you complete the internship at the same place you do your residency.