You're pretty helpful to a valid question regarless of how "necro" it is...
The Wikipedia page quoted here already states the differences between the C8A2 and the C8A3, although a little cryptic. Read:
http://en.wikipedia.org/wiki/Colt_Canada_C7_rifle under the C8 section "Other C8 variants" you will find this, "The new version will be designated the C8A3. The C8A3 will combine the C8FTHB with the upgrades found on the C7A2. These improvements include green furniture, ambidextrous fire selector, magazine catch, and an extended ambidextrous charging handle." Just read the feature headings and it should become clear.