You need one of the Raspi cases (1st item below), and 1 intermediate stacking plate (2nd item below) for a second RPi. So, for 3 RPi: 1 case, 2 intermediate plates.
The bottom plate of the case goes at the bottom of the stack. The top plate of the case way at the top of the stack. The products come with all required parts (nuts, bolts, standoffs, etc.)
- Raspi case, stackable EUR 9.00
- intermediate stacking plate for above EUR 6.85
- Netgear GS105, 5-port Gigabit switch EUR 15.90
- 0.25m CAT6 Ethernet patch cables (set of 5) EUR 9.90
- Anker PowerPort5 40W USB multi-port charger EUR 20.00
- Anker 4-pack PowerLine micro USB cable 0.3m EUR 9.00
it would be nice if the switch was also part of the stack somehow...