Coat one 9-inch square pan and two 9-by-13-inch pans with cooking spray. Prepare the cake mixes as directed; pour 1 1/2 cups batter into the square pan and divide the rest between the other two pans. Bake at 350 degrees F, 15 minutes for the square cake and 20 to 25 minutes for the 9-by-13-inch cakes.
Let the cakes cool slightly; remove to racks to cool completely. Stack the 9-by-13-inch cakes on a cutting board with frosting in between them. Cover the sides of the cakes with a thin layer of frosting, filling in any gaps between the layers. Freeze until firm, about 1 hour.
Make the field: Trim off 3 1/2 inches from one side of the square cake; use the larger piece for the field. Reserve the trimmings.
Put 1/2 cup coconut in a resealable plastic bag; add a few drops of green food coloring and massage until tinted. Repeat with 1/4 cup coconut, using red and yellow food coloring to tint it orange (or use your own team colors).
Place the rectangular field on a large cake board (about 10 by 13 inches). Spread with frosting, then cover most of the field with the green coconut and the end zones with the orange coconut, pressing the coconut into the frosting.
Make the goalposts: Trim 2 lollipop sticks to 2 inches. Melt the candy melts in the microwave; cool slightly, then transfer to a resealable plastic bag and snip a corner. Pipe two goalpost tops onto parchment; add a stick to each and pipe over them to secure. Let dry.
Cut the 9-by-13-inch layer cake lengthwise into thirds. Cut each strip of cake on the diagonal to make the sloped sides of the stadium; reserve the trimmings. Cut the center strip of cake in half to form the short ends of the stadium.
Arrange the cake strips around the field, trimming as needed. Remove the field and set aside. Round out the corners of the stadium using a serrated knife to form an oval shape; reserve the trimmings.
Crumble all of the reserved cake trimmings and mix in just enough frosting to moisten. Clump the mixture together with your fingers.
With damp hands, use the crumbled cake mixture to fill in the gaps and form a uniform oval stadium with sloped sides. Trim the cake board as needed so it doesn?t show.
Frost the entire stadium. Coat the sloped parts with the nonpareils to look like fans. Press the remaining white coconut around the outside of the cake.
Return the field to the stadium and pipe yard lines across it with frosting. Add the goalposts. Pipe the remaining frosting around the top edges of the cake.