Bake your cake according to the box cake instructions.
Cool cake completely.
Use wax paper to draw a number 3 you will use as a stencil to trace onto your cake. Cut out the number 3 with scissors and place it on top of your cake. Trace the number 3 onto the top of your chocolate cake with a sharp knife.
Carefully dig out the inside of your number three cake. Just dig out a shallow small amount of the chocolate cake.
Scrape out the cream from 6-10 Oreo cookies. Put the black cookie part of the Oreos in a ziploc bag and crush them, or crush them with your food processor. Sprinkle the cookie crumbs into the cut-out 3 on your cake.
Mix green food coloring with your white frosting. Or mix one drop of blue, yellow, and brown together for the same color seen in my photos. Put the icing in a piping bag with a star tip or grass tip and cover the surface of your cake surrounding the 3.
Decorate the top of the cake with small dinosaur toys and plastic leaves. Optionally, add mini chocolate eggs as 'dino eggs'.