Depends on the state your in. Minnesota only allows towing of an additional trailer behind a 5th wheel (which is odd). When we moved down to Arkansas I checked the laws in Iowa, Missouri, and Arkansas, and all three allow triples with almost no restrictions. I really didnt want to make an extra trip just for the boat, so I hooked it behind the Jeep Cherokee that I towed behind the Motorhome. I had my wife following so figured if Minnesota made me park it somewhere, we would just stop across the Iowa border and go back for it. I passed several Minnesota LEO's and they just looked the other way...
Just for clarification, it was a 18 foot fishing boat with a 80 hp outboard. I didnt even know it was there and no one gave me a second look. Just make sure the tow vehicle is heavier than the combined towing weight by a factor of at least 1.5. Brakes on at least one of the trailers would be wise. But if your state says no to triple tow, I probably wouldnt try it. As the MH was over 20K pounds, I never had brakes on the Jeep, and none on the boat either, but the MH stopped about the same regardless.
P.S. You see a lot of trailers in the south with a trailer ball. Guess it just depends on how how restrictive your particular state is at keeping a thumb on you. I was on the fire dept here and never heard of a wreck (or read of any either). It might be that anyone willing to attempt it is a safe enough driver to drive accordingly. Cant say what MN's deal is other than the thumb thing. Its curvy and hilly as hell here, and flat as a pancake there.