It's high-time I knew more about this man. Which 3 of his albums should I buy as a good introduction for a Christy Moore beginner?

Live in Dublin 2006: either the double CD or the DVD or both. I'd go with the latter, if you only want one, simply because its possibly the best folk DVD I've ever seen. Musical content is more or less the same, running order is different. Depends if you like to see the show as well as just hear it.