E1, M3: Toxin Refinery
This map has several secret places, including one that leads to a separate exit to get to a secret level!
Secret 1
Press the switch in this picture to open up a secret stairway to the yellow key room.
You can find a hidden elevator platform by running up the stairway and sprinting to the left. Here you will find a soulsphere.
Secret 2
The same as secret 1, but on the right side of the yellow key room instead of the left.
Secrets 3 and 4
Inside secret 2, there is a tunnel in the slime that leads to a switch. The switch raises a platform in the slime pit at the starting area of this level.
Next to the switch in secret 3 is a section of wall that looks a bit different. This is a secret door that opens to reveal a chain gun.
Secret 5
In the room past the blue key door is a slightly inset wall. This is a secret door that leads to a bunch of goodies.
Secret 6
In the starting room, there is a pathway over the slime, which appears when you toggle the switch in secret 3. This leads to the secret level exit.
Secret 7
The wall opposite the secret exit switch is a final Map 3 secret that leads to a box of rockets.
Here’s a video walkthrough of Doom Episode 1, Map 3. I initially went to the right instead of the left in the video, so you see secrets 2, 3, and 4 of this post before secret 1.