To kill 3 innocents:
Go to a city with a number of people on the outskirts (i.e. outside the walls). Quickly stab/kill 3 people and RUN for the hills until you are "completely" outside the city limits.
You'll always here the HALT, HALT but you can out run the guards. The Dark Brotherhood will contact you in a few days.
(If you got away without being stopped you can re-enter that city and not be bothered by the guards).
For some good info go here http://www.izhtex.com/tes2/ and then to the section labeled "OK so I'm Pond Scum".