2020-05-03 01:52:08 temperature.c:450:temp_statistics_show: pcb temp 42~56 chip temp 65~80
2020-05-03 02:22:09 thread.c:204:calc_hashrate_avg: avg rate is 27625.39 in 30 mins
2020-05-03 02:22:09 temperature.c:450:temp_statistics_show: pcb temp 42~56 chip temp 64~79
Your temps are pretty high, chain 2 does show less ASICs than it should but that's not your only issue, your temps are pretty high forcing the miner to lower the voltage/frequency which may cause it to be unstable and then reboot/stop mining. When you remove the third board (Chain 2) the other two boards will cool down and there is a good chance they will mine on without an issue and you will think that chain 2 was the only problem when actually you have a heat problem.
I suggest you set the fans at 100% and see if the miner works more stable with these settings if you don't know how to set a fixed speed for those fans let me know I'll show you.