buggy awk regex handling?

[kaltheat]

Hi,

I tried to replace three letters with three letters by awk using the sub-routine.
I assumed that my regular expression does mean the following:

match if three letters of any letter of alphabet occurs anywhere in input

$ echo AbC | awk '{sub(/[[:alpha:]]{3}/,"cBa"); print;}'
AbC

As you can see the result was unexpected.
When I try doing it for at least one letter, it works:

$ echo AbC | awk '{sub(/[[:alpha:]]+/,"cBa"); print;}'
cBa

Same problem without macro:

$ echo AbC | awk '{sub(/[A-Za-z]{3}/,"cBa"); print;}'
AbC

$ echo AbC | awk '{sub(/[A-Za-z]+/,"cBa"); print;}'
cBa

I thought that it might have something to do with the curly braces. But escaping
them doesn't do the trick.

What am I doing wrong?
Or is awk buggy?

Regards,
kaltheat