Once you cross second bridge, you'll see a little courtyard on your right, the one that contains grenade launcher inside. So, two secrets are around this yard, you just need to walk outside its wall.
And the third secret is just before level exit. There are two ziggurats before the final door, you need the one on your right. If you walk between this ziggurat and giant blue wall you'll notice bright square on ziggurat's wall. Shoot it with any weapon.

thank you!