From the start, go down the stairs and drop into the "moat". Follow it left for a while, until you reach a pair of overhanging green structures. An unreachable blue combat armor vest is visible in a nook underneath the western one. Press the use key on the brown wall underneath the nook that does not contain the blue armor, and both nooks will lower. Stepping into either nook before they rise again will register the secret.
From secret #1, continue east along the moat until you reach a wooden chamber with several lion-faced pillars in it. One of them has blood splattered on its side; open it to reveal a teleporter to a megasphere. Note: since the teleport pad itself is the secret, you'll actually need to use the idclipcheat code to register this secret, as otherwise you'll get whisked away before the secret can register.
From the megasphere, head west down some stairs, then turn left and go straight up another set until you hit a wall. The green-tinged wall to your left opens up, revealing a teleporter to a combat armor vest.
In the room south of secret #3, there's an opening in the east to a vaguely "3"-shaped hall. At the north end of this corridor, there is another green-tinged wall that can be opened for a secret.
Secret 2, the teleporter to the megasphere, cannot be registered without cheating.