Combine all 3 boxes of mixes together, mixing well. Add the oil and eggs, and any other ingredients that your particular cake mix calls for. Add water until the resulting batter is fairly thick yet. This doesn't seem to be too critical, except if it is too thick it seems to burn easier. Preheat the Dutch oven slightly and oil up good. Add the batter. Drain the pineapple juice and spread the pineapple out evenly over the top of the batter. Place the cover on the oven. Use a very small amount of coals on the bottom, about four or five charcoal briquettes worth. Cover the oven top with coals, and bake for about 30 minutes. Replenish the coals on top if needed.