You must first let the russians shoot a hole into the wall. After that, you can comfortably climb up to the green gem.
Ride back to the switch house and toggle switch #3. Now take another round. This time you will be attacked by a bazooka as soon as you come to the place opposite from the room with the z-shaped pool. The commies will fire a hole into the trails and the wall but you will survive. Now return there, clear the area and get the green gem. You can use the new hole to dive into the z-shaped pool.