That setup will work fine with a master switch (where line and load is - right most switch box in your diagram) and two add-on switches. Wire it the same as line-switch-load-switch-1 in the FAQ and wire the second add-on in parallel to the first one. You will not need to use any other wires in your boxes than those in the diagram above. Also note you will only need two wires between each of the switch boxes instead of the three that currently are used.