These three brothers served in Europe during the war. None appeared to be career military before or after the war. So each Flanagan brother should have registered for the draft. Try as I might, I could not find a draft registration card for the oldest brother Michael Flanagan.
I finished reading a book today that I want to recommend to anyone who is researching their U.S. World War I ancestors, including females. Although short (95 pages) the book is jam-packed with information about record collections, repositories, and search strategies.