Box 1 I believe you just want to use the 3-wire, the black/white one is a black insulation coated white so you might want to wrap some white tape around it to ensure it is identified as white. Switch hooks to white and black from it, no load. The whites from the other wires just get joined together.
Box 2 I believe has the power. So, black and white from that gets connected to the black and white from the two 3-wire. Switch gets connected to those as well, no load.
Box 3 I believe has the load. Black and white from the 3-wire goes to line and neutral of the switch. Black and white from the 2-wire goes to load and neutral.
It is possible to wire it so one of the other switches can directly control the load. This could be done to ensure it has control no matter what else might happen to the other switches or z-wave network. The one controlling the load can be slightly more responsive using associations as well. If you want this, the red wire gets used.