Mutagen- Samurai 3
After you have some fun with the Sentinel's hand, go down the middle corridor and up the elevator. Once you are off the lift look to your left and you will see the mutagen on the right side of the stairs. It's in the same room where you have to control a teleporter.
Mutagen- Art of War 3
Art of War 3.jpg
Once you free Wraith, there will be a cut scene, when you get control again turn around. The mutagen is on the left side of the cell that was holding Wraith.
Chapter 4: Mardi Gras
Chapter 4, Checkpoint #1 (The Corbeau Casino)
Mutagen- Blood Rage 3
Blood Rage 3.jpg
There will be a cut scene with Gambit and you will be introduced to the Assassin. Once you kill the Assassin walk through the doorway that the Assassin and Gambit came out of during the cut scene. This is NOT the door that you kick open. It will be to your far right at the top of the stairs. Right of the door you kick open to follow Gambit. Make sure to get this before going out of the room where you fought your first Assassin or else you will have to replay the level.
Chapter 4, Checkpoint #2 (High Rollers Lounge)
Mutagen- Savage 3
(Hehe...Okay so I accidentally got the mutagen in this one as well..again you see where I'm talking about.)
After using your claws to scale the side of a building, two Assassin's will break through the glass windows to your left. Kill the Assassin's and then jump into the window they broke out of and you will see the mutagen.