The MY has a much more comfortable rear seat than the M3. Yes, it's about 1" narrower, but:
1. The seat sits much higher and the floor is lower = knees not in your chest.
2. The seats have two reclined positions (max recline for only shorter passengers though).
3. The front seats are on risers = way, way more foot room for the back seat.
4. The roof is higher = more head room (M3 has a lot already though)
5. No mid-roof crossbar = open feeling.
Three average adults are uncomfortable in the back of either one for different reasons, but two adults are way more comfortable in the back of an MY.