Match First Sequence in Regular Expression?

[Roger L. Cauvin]

Say I have some string that begins with an arbitrary sequence of characters 
and then alternates repeating the letters 'a' and 'b' any number of times, 
e.g.

"xyz123aaabbaabbbbababbbbaaabb"

I'm looking for a regular expression that matches the first, and only the 
first, sequence of the letter 'a', and only if the length of the sequence is 
exactly 3.

Does such a regular expression exist?  If so, any ideas as to what it could 
be?

-- 
Roger L. Cauvin
nospam_roger at cauvin.org (omit the "nospam_" part)
Cauvin, Inc.
Product Management / Market Research
http://www.cauvin-inc.com