I couldn't find a decent review of this jacket on the web, so I here you go. I'm 5'10, 145lbs and I end up in medium Pearl Izumi jacket. The chest is still slightly baggy, but the arms and torso were too short on the next size down. For reference, I usually wear a small or Euro XS jersey.
The 3x1 is basically a wind-proof insulated jacket inside a non-insulated, wind-proof, very water-resistant soft shell jacket. They are held together via a button snap on the sleeves and collar, it works well and is simple. Once the jacket is on, it feels like a one-piece jacket.