Preheat oven to 350°F. Line a 9x13 standard cake pan with aluminum foil over lapping edges to use as handles to remove cake from pan after cooling. Spray sides and bottom with cooking spray. Set aside. Beat cake mix, sour cream, eggs, oil, and 3 (mashed) bananas on low speed just until moistened. Scrape sides of bowl and continue to beat for 2 minutes on med. speed. Pour into prepared pan. Bake for approx. 35 minutes or until cake is set and very brown on top. Remove to wire rack to cool for at least 1 hour.
While your cake is baking, put a large bowl and whisk or beaters in the freezer for 15 minutes. (This will help stabilize your whipping cream as you are beating it.) Remove from freezer and pour whipping cream into chilled bowl and beat on med/high speed for 10 minutes. Add 2 1/2 cups of powdered sugar and continue to beat until very stiff peaks form. Transfer to another large bowl and refrigerate until ready to use.
In the same bowl that you used to beat your whipping cream, (no need to wash it) add your pudding mixes and milk and beat for 2 minutes making sure all pudding is mixed thoroughly. Take 1 1/2 cups of your whipping cream and add to pudding mixture and beat on low speed for 1 minute.
Peel remaining bananas, slice lengthwise and remove any 'bad' spots. Slice into 1/2 inch pieces and add to pudding mixture. Fold in with a spoon.
You are now ready to assemble your delicious Banana Trifle!!! Remove foil from cooled cake and slice into 1 inch strips. In opposite direction, slice into 1 inch strips. (Basically, you'll have cubes of cake.) In a 3 quart trifle bowl (or serving dish of your choice) put approx. 2 cups of pudding covering bottoms. Add cake cubes to completely cover pudding (approx. 1/4 of your cake). Push into pudding gently. Do the same with whipping cream. If graham cracker crumbs are used, sprinkle over top of whipping cream. Repeat in same fashion for another layer. (About 1/2 of your cake and other ingredients will be left.)