I want to add the string, '0', in front of every string in column 'a' if the values in the columns 'a' have a string length of 3. How do I do this?

code: import pandas as pd

df = pd.DataFrame({'a': ['abc', 'abc', 'abcd'],'b': ['abc', 'abcde', 'abcd'], 'c': ['abcd','abc', 'abcd']}) display(df)