Sheepskin jackets are the mutts nuts as far as warmth goes, but my feeling is that for a M3W you need to look at serious waterproofing also....sitting in a puddle of water is not fun.
Space in the 3W is tight so you might like to consider quality waterproof biking gear that is also warm without being bulky..(safe too).
I can fully recommend Rukka..but quality comes at a price, so perhaps you should try a more budget solution until you have a bit more experience with how you use the car....eg borrow a sheepskin to see for yourself, for example.
Good luck
