Hello,

I have a picklist field that contains values A-Z. I need to write a validation rule that says if the Picklist is set to A, B or C then the same Picklist can't be changed to the value X but can be changed to other values. 

OR(
AND(
ISPICKVAL(PRIORVALUE( VPM_Field__c ),"A"), NOT(ISPICKVAL(VPM_Field__c,"X))
),
AND(
ISPICKVAL(PRIORVALUE(VPM_Field__c),"B"), NOT(ISPICKVAL(VPM_Field__c,"X")))
),
AND(
ISPICKVAL(PRIORVALUE(VPM_Field__c),"C"), NOT(ISPICKVAL(VPM_Field__c,"X")))
)


The above is what i have but it doesnt work 

Thanks