to start the axis you need a speed setpoint and the OFF1 command - so you parametrize p840 to a digital input.
Why you need OFF3? If you need it for an emergency stop, you can use the SAFETY functions (for example SS1).
How you want to quit an error? If you would have a HMI, you can use the function "HMI- direct access" to visualize errors, change speeds and so on. What hapen, when one drive has an error? Do you need additional logic - then you can use free blocks, or better: DCC.
Note: With HMI- direct access you need a digital input to stop the axis, because a HMI is only class2 master, so a Profibus-/net disconnection wouldn`t be evaluated.