I have read a lot on archery form, and really don't know of much out there in the way of books that have much space devoted to 3 under. For example, Shooting the Stickbow, by Anthony Camera, which is a book I refer to often to answer my own questions, has 1/2 page out of 422 pages of text that discusses 3 under. Total Archery, Inside the Archer, by Kisik Lee, probably the most technical book I have read on archery form, has no text specifically devoted to 3 under.
If you find one that has more, please let me know.
In the mean time, those of us who shoot 3 under would be happy to discuss it with you, if there is any way we can help.