You just do like the first add-on. You're
wiring in series. From one wire on the
sensor to a screw terminal on the magnetic
switch, a wire from the second screw
terminal on the first magnetic switch to a
screw terminal on the second switch. A wire
from the second screw terminal of the second
magnetic switch to a screw terminal on the
third magnetic switch, and the other wire
from the sensor going to the second screw
terminal of the third magnetic switch. I
hope this makes sense to you and that it is
of help.
