Using lists, write a Python program to add 'ing' at the end of a given string. If the given string already ends with 'ing', add 'ly' instead. If the string length of the given string is less than 3, leave it unchanged.

Sample string: 'abc'

Expected output: 'abcing'

Sample string: 'string'

Expected output: 'stringly'

Sample string: 'oh'

Expected output: 'oh'