Green Skull:
to get the first skull, go to the first floor of the pyramid and then go to the second door on the right, in that room there should be one vase, press use on the vase and then leave the pyramid. when you leave the pyramid, the left temple that is furthest forward from the pyramid from the should be open, when you go in the left temple the green skull should be on the far right corner, this skull goes on the middle pedestal at the bottom of the pyramid

Red Skull (not the marvel villain)
to get the red skull first go to Germany the fountain at the bottom of the pyramid, the red skull is in the top of the fountain on the side that faces the pedestal room in the far left corner. this skull goes on the left pedestal at the bottom of the pyramid.

Blue Skull:
to get the blue skull, go to the tallest obelisk at the water. at the bottom of the water at the far right corner of the outcrop obelisk is on is where the blue skull should be. this skull goes on the right pedestal at the bottom of the pyramid.

when you find all the skulls, you unlock the purple portal secret that will appear above the pyramid.