The simplest thing is to create three separate cells, one for each element, as follows:
$this->Cell(30, 10, 'Description', 1, false, 'C', 0, '', 0, false, 'T', 'M');
$this->Cell(130, 10, 'Page '.$this->getAliasNumPage().'/'.$this->getAliasNbPages(), 1, false, 'C', 0, '', 0, false, 'T', 'C');
$this->Cell(30, 10, 'Reference number', 1, false, 'C', 0, '', 0, false, 'T', 'M');
( $border '1', , ). , , .