river-wear: Having just done battle with algae myself, I feel I can pass along a few thoughts.

With FC of 3 and CYA of 40, your shock level is 15. Using the Bleach Calculator for your size pool of 27,000, it calculates that you would have needed 5.4 gallons of bleach to raise your FC 12 ppm (from 3 to 15).

Shock your pool, continue to brush, run the filter 24/7. Test as often as you can, 2 or even 3 times a day if you can manage it and bring that FC level back up to shock each time. It'll clear.

As to why it happened when your #s look good...maybe CanuckPool is right. Otherwise, algae spores are everywhere, even in "perfect" pools. Algae happens .