applyFromArray()
is a method of the PHPExcel_Cell object, not of the cell value (which is a simple PHP datatype unless the cell contains rich text).... so you need to call it against the cell
$objWorksheet->getCellByColumnAndRow(3, $row)->applyFromArray;
but if you need a style based on the value of the cell, then you should use conditional styles. See section 4.6.23 of the developer documentation ("Conditional formatting a cell") for details, and look at 08conditionalformatting.php in the Examples
EDIT
$objConditionalStyle = new PHPExcel_Style_Conditional();
$objConditionalStyle->setConditionType(PHPExcel_Style_Conditional::CONDITION_CELLIS)
->setOperatorType(PHPExcel_Style_Conditional::OPERATOR_LESSTHAN)
->addCondition('12');
$objConditionalStyle->getStyle()->getFont()->getColor()->setRGB('FF0000');
$objConditionalStyle->getStyle()->getFont()->setBold(true);
$conditionalStyles = $objWorksheet->getStyle('A3')
->getConditionalStyles();
array_push($conditionalStyles, $objConditionalStyle);
$objPHPExcel->getActiveSheet()->getStyle('A3')
->setConditionalStyles($conditionalStyles);