MySQL REGEXP repeating characters

How can I find strings with individual repeating characters (for example 3 times)?

This works but I don't want rewrite every alpha characters (a,b,c,d,e...):

... REGEXP '(a){3}|(b){3}|(c){3}|(d){3}|(e){3}|(f){3}...' ...

Thank you very much

r = '(\w)\1{2}`

finds any alphanumeric character that is repeated (at least) three times, matching the first three.

How about repeating a capture group?

r = '([abc])\1\1'

'(abc){3}'

Example:

> echo 'bbbabcabcabcaaabbbccc' | egrep -o '(abc){3}'
abcabcabc