R. Nehorai says: These three, added to three; "then you shall add for yourself" — nine; "another three" — twelve.
Abba Shaul says: Three added to three, "another three" — nine; "on the three" — twelve; "these" — fifteen....
(Devarim 19:10) "And innocent blood shall not be shed in the midst of your land that the L-rd your G-d gives to you as an inheritance": Beth-din is hereby being exhorted to this end.