$arFiles array (size=6)
  0 => string '101858 - xxxxx.jpg' (length=64)
  1 => string '1233443zzzzzzzz8674zz.jpg' (length=62)
  2 => string '210 - sssss.jpg' (length=61)
  3 => string '686117 - qwe.png' (length=83)
  4 => string '713531 - asd.jpg' (length=125)
  5 => string 'qweefsds' (length=74)

$arIDs=array();
foreach ($arFiles as $key=>$value) {
    $id = preg_match('/^\d{3,}/', $value, $matches[$key]); 
    if ($id) {
        $arIDs[]=$matches[$key][0];
    }
}
var_dump($arIDs);

$arIDs array (size=5)
  0 => string '101858' (length=6)
  1 => string '123443' (length=6)
  2 => string '210' (length=3)
  3 => string '686117' (length=6)
  4 => string '713531' (length=6)