Yeah, with the 800 PSU you can't run more than two boards. My former S7 used up to 1400 watt.
Check if all 3 boards are connected properly and the diods are on. If one isn't bright red there u have the faulty board. If one board is not connect you can try disconnecting one board and then connecting the formerly not connected board to see if it's still working. If it is working you have to buy another PSU and then u can connect all 3 boards and run it at full capacity. If not, u have a 3 TH miner