Sound to me like you are planning on putting a lot of
weight on that third part of your train. You didn't mention the size of the "Boat", but a car hauler usually has a car on it and I really doubt if the back end of even a BigFoot would welcome another 3500-5000 lbs on it's tail.
On top of that, if it weighs over 1500 lbs, (1000 lbs in some venues) you will have to have
brakes on the second trailer as well.
And I am trusting that you have a Big A$$ Diesel to pull all of this with.....
As far as driving in California, you would have to meet all CA-DMV requirements to drive it in the Golden state. But then you would have to figure which states and Provinces you could pull it through to get here first.
And yes, what you want to do "May" be legal, but I can guarantee that you will get lots of attention you don't want when doing so. Posting a sign that sez "Yes, It's legal" sounds like it could be someone with an attitude problem, one of CHP's favorite customers.....
Maybe you should get the BigFoot first and find out if even that part works for you...