Grabbed the strategy from this article (http://www.kotaku.com.au/2014/02/tips-for-playing-threes-the...)! Basically there is a deck of 12 cards (four 1's, four 2's, four 3's) that is shuffled, drawn from, and reshuffled when the deck is empty. Originally I just had it populate randomly from the same distribution, but it resulted in a startlingly high frequency of terrible boards with like 10 red tiles floating around and no blues.
As mentioned in an earlier comment and in the article, the actual iOS game also inserts higher-number cards (6, 12, 24, ...) into the stack occasionally, though I haven't added that element to this version yet.
As mentioned in an earlier comment and in the article, the actual iOS game also inserts higher-number cards (6, 12, 24, ...) into the stack occasionally, though I haven't added that element to this version yet.