Trigger Happy ... Not

Hello Everyone,  I am still learning Sql and need some help with a query.  On the master table, I have an audit log. I have a trigger set up to run after an update.  I only want to perform an update if the value is not true or one record updated will result on 1800 records in you audit log.  I tried to disable and enable the trigger from within the trigger unsuccessfully. So now  I am trying an If statement, in a trigger Maybe this is where you can help ? 
I get the following error  
 
Msg 512, Level 16, State 1, Line 1
Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression.

Extract from Trigger.

If (Select Discprency from Master_Table
where ((Pro1 <> Pro2 and Pro1 <> Pro3) or
(Pro2 <> Pro1 and Pro2 <> Pro3) or
(Pro3 <> Pro1 and Pro3 <> Pro2) or
(Pro1 = Pro2 and Pro1 <> Pro3 and Pro2 <> Pro3) or
(Pro1 = Pro3 and Pro1 <> Pro2 and Pro2 <> Pro3) or
(Pro2 = Pro3 and Pro1 <> Pro2 and Pro3 <> Pro1))) = 0
Begin
Update Master_Table
set Discprency = '1'
where Discprency = 0
End
Else
Begin
Update Master_Table
set Discprency = '0'
where Discprency = 1
End


 

Thanks for your time.