Actually you can use all three ports simultaneously, but between two of them forwarding is not possible.
The port which is selected at Forwarding -> Set As Uplink Port ca communicate to the other two ports.
/interface detect-internet set detect-interface-list=none internet-interface-list=none lan-interface-list=none wan-interface-list=none
/system clock set time-zone-autodetect=no
/ip cloud set update-time=no