Heat oven to 350°F (325°F for dark or nonstick pans). Grease or spray bottoms and sides of four 8-inch square pans. In large bowl, beat 1 cake mix, 1 cup of the water, 1/2 cup of the oil and 3 eggs using electric mixer on low speed 30 seconds, then on medium speed 2 minutes, scraping bowl occasionally. Divide batter between 2 pans. Bake 2 pans at a time for 23 to 29 minutes (26 to 34 minutes for dark or nonstick pan) or until toothpick inserted in center comes out clean. Cool 10 minutes; remove from pans to cooling racks. Cool completely, about 30 minutes. Repeat with remaining cake mix, water, oil and eggs. Chill cooled cake in freezer 45 minutes before cutting, to reduce crumbs.