But do know that I have cruised each December (not Christmas week) for three years and have never been upgraded. We were cat. 5 once and 6 twice. My table mates each time were also cat. 6's and they were not upgraded. It would be great if you get one but please don't think because you are going in Nov. or Dec. and are in a popular cat. you have a better chance. Most upgrades I am aware of were from cat. 11. And I would think the chance of getting a many thousand $ upgrade to a cat. 3 is just about impossible. A cat. 3 for three people runs around $10,000, a cat. 7 for three people under 4,000.
That said though I do wish you luck, just don't be let down if it does not happen.