Programmatically format integer field display
You can do this using the following code:

$drupal_version = (int)$drupal_version; // make sure this is an int
$parts = array();
preg_match('/^(\d{1})(\d{2})(\d+)$/', "$drupal_version", $parts);
array_shift($parts); // get rid of the first element in $parts which is the whole matched string
$formatted_drupal_version = implode($parts, '.');
echo "Drupal version: " . $formatted_drupal_version;


As examples, these integers will turn into the following strings:
7411 => 7.41.1
74110 => 7.41.10

Which means, that this code, will work parsing the provided integer from left to right, and collecting, 1 digit for the major version, 2 digits for the medium version, and all the remaining digits for the minor version.

Hope this helps!

P.S.: You question is a bit misleading, as @No Sssweat says, you cannot format an integer like that. That is why string parsing is the only way to provide you with the desired result.