Do you get the same problem if you do:
$query=$this->mxpdo->newQuery('myclass');
$query->where(array(
'col1' => $postvars['a'],
'col2' => $postvars['b'],
'col3:LIKE' => '%'.$postvars['c'].'%',
));
$cdata = $this->mxpdo->getCollection('myclass',$query);