Heat oven to 325°F (for all pans). Spray bottoms only of two 8x4-inch loaf pans with baking spray with flour.
In large bowl, beat cake mix, eggs, coconut milk and rum extract with electric mixer on low speed 30 seconds, then on medium speed 2 minutes, scraping bowl occasionally. Stir in coconut and pineapple. Pour into pans.
Bake 58 to 63 minutes or until toothpick inserted in center comes out clean. Cool 10 minutes. Run knife around sides of pans to loosen cakes; remove from pans to cooling racks. Cool 50 minutes.
In small bowl, mix powdered sugar and reserved 3 tablespoons pineapple juice. Poke tops of cakes every inch with toothpick. Pour sugar mixture over cakes. Cut each cake into 8 thick slices, using serrated knife. Store loosely covered.