need simplified update query

[bruno matthys]

Hi folks
I'd like to squeeze the following 3 update statements into one query (PHP is used).

$res1 = "UPDATE tablename SET res_id1 = 0 WHERE res_id1 = '". $_SESSION['res_id'] ."'";
$res2 = "UPDATE tablename SET res_id2 = 0 WHERE res_id2 = '". $_SESSION['res_id'] ."'";
$res3 = "UPDATE tablename SET res_id3 = 0 WHERE res_id3 = '". $_SESSION['res_id'] ."'";

Basicly I'm sending a value which is checked against 3 cols in the table; if the value sent equals the value in col 1 (or col 2 or col 3) then the value in col1, 2, 3 should be set to 0.
I've been trying to get IF, ISNULL, CASE to work but I'm new with control flow like this - and I haven't found a decent tutorial for it yet (if you have a link to one plz post it).
enlighten me :-)