Correct syntax Union select

[Josephsch]

Hi could somone kindly correct this for me?
$result = mysql_query("SELECT Distinct `Category` FROM `Products`, as Cat, Union SELECT Distinct `Category2` FROM `Products`, as Cat, Union SELECT Distinct `Category3` FROM `Products`, as Cat ")

so that i have a result of all unique categories regardless of if they are in category1,2 or 3 in one row.

Thanks.