<?php
$s = '<sup> 1, 2, 3- 124</sup> 2345';
echo preg_replace('~(?:\G(?!\A)[^\d<]*|<sup>[^\d<]*)\K\d+~', '{{{$0}}}', $s);
PD9waHAKCiRzID0gJzxzdXA+IDEsIDIsIDMtIDEyNDwvc3VwPiAyMzQ1JzsKZWNobyBwcmVnX3JlcGxhY2UoJ34oPzpcRyg/IVxBKVteXGQ8XSp8PHN1cD5bXlxkPF0qKVxLXGQrficsICd7e3skMH19fScsICRzKTs=