well now i solved it - was just a tiny little thing, but if no one helps these things are sometimes hard to find

here`s the code for the correct where clause

Code:
 $where_str .= " AND (pac.class_of_absorbtion=:absoID
 OR puc.class_of_absorbtion_second=:absoID
OR pic.class_of_absorbtion_third=:absoID)";