Priority problem

I would set up just one register and allow the user to choose only one value. Possibly a drop down selector or multiple pushbuttons pushing different values to the same tag. There are only 6 possible combinations assuming that each agitator must receive a priority. See table below:
Agitator
Priorities
Register Value A B C
1 1 2 3
2 1 3 2
3 2 1 3
4 3 1 2
5 2 3 1
6 3 2 1

The user can only choose from one of the six options. Use EQU instructions in your PLC to set the priority values for each agitator. This guarantees that there will never be two units with the same priority level.

Just have some bool tags in the processor. One goes true when 'Priority 1' is assigned, one goes true when 'Priority 2' is assigned. While you don't need one for the third priority, you might want to include it anyway for consistency and in case of future expansion.

Link the 'Priority X' assigned tags to the HMI to disable the operators used to select them so you can't pick more than one.

I always did this with simpe arithmetic. 1 + 2 + 3 = 6. If the operator assigned 2 as lead and 1 as lag then 6 - 2 - 1 = 3 and unit 3 is the standby.

Thanks... I needed that