How to set IN values on an update (SQL Server)?
Update apotable
   set price1 = 0, 
       price2 = 0,
       price3 = 0
where po_number IN ('x', 'y', 'z');


Note that numbers should not be enclosed in single quotes 0 is a number, '0.00' is a character literal (aka "string"). 

There is no need to search the web for the syntax of a SQL statement.
That is all documented in the manual: 
https://msdn.microsoft.com/en-us/library/ms177523.aspx