GO TO JAPAN!
Artist: Shohei Otomo
They split equally between all paths.
So, one zombie stays, one takes path one, and you add two more zombies to take path 2 and 3 (one each).
You said your survivors are "in buildings" - not sure if this applies, but just in case, remember that from the street, you do have LOS into the first room of any adjacent building as long as the door is open. (Lots of indoor/outdoor LOS questions resolved here.)
Like I said, not sure if it applies, but just in case.