Regex to find triple characters

[Gribouillis]

This perhaps

>>> import re
>>> p = re.compile(r'(\w)(?<!\1\1)\1\1(?!\1)')
>>> 
>>> [m.group() for m in p.finditer('ab999thc7')]
['999']
>>> [m.group() for m in p.finditer('abddddthc7')]
[]
>>> [m.group() for m in p.finditer("AAAbbcDDDEDGGGG")]
['AAA', 'DDD']