It really depends on how your county/state handle the registration process. Here in Alabama, we dont title outboards or trailers. So when you go to register, you are basically just registering the hull and listing the motor information for informational purposes. No trailer info on it from what I can remember. If you are in Cali, I imagine they probably will try to make you register them all. I'd think the main thing is to have correct serial number of all 3 as well as the Hull ID number in the bill of sale. Make sure the seller's info is exactly how it reads on the current registration they have. If they require titles or Certificates of Origin for them you will need those signed over as well.