<?php

// your code goes here

$array =[ 
  [
    'col_1' => 'one',
    'col_2' => 'two'
  ],
  [
    'col_5' => 'five',
    'col_3' => 'three'
  ]
];

$key = -1;

foreach($array as $k => $el) {
	if (array_search('three', $el) !== false) {
		$key = $k;
		break;
	}
}
echo $key;