<?php

$content = <<< LOL
ID:::value1:::value2:::value3:::
1:::My:::first:::line:::
2:::My:::second:::line:::
3:::Your:::third:::line:::
LOL;
$content = preg_replace('/(^ID:.*\S)/im', '$1value4:::', $content);
$content = preg_replace('/(^\d+.*\S)/im', '$1:::', $content);
echo  $content;