Triple towing

This is illegal in many states.

Check the laws in the state you wish to do this.

Although we're from Virginia and it's not legal here, I know double towing is legal in Michigan and quite a few other states, especially west. Although I've seen triple towing semi's out west, I've never seen any RVs doing that.

First though you'd better verify your trailer's hitch is up to towing anything at all. Most of them are really not good enough for even a bicycle or two. Everyone I've seen towing a boat behind a trailer had a custom hitch welded onto their frames.

Safety is main issue along with your insurance carrier...will they cover a triple tow...almost all eastern states do not allow triple tow...Google rv towing laws by state

IIRC some states allow only specific combos to be towed. PU/5er/boat
for example.
bumpy

Even if double towing is legal where you are going, with a 30' 5er you are probably going to be over the 65' max length in many states.

Recreational doubles legal here in Michigan, triple's no way.

Semantics aside it seems your asking about a double tow which is perfectly legal in MI as well as about 30 other states. As long as your hitch and truck are up to the task you can do it.
I'm not sure I'd try it with a pontoon however. From what I've heard those things are a chore to tow solo. The have a rather high center of gravity compared to other boat types. They catch a lot of side wind and a lot of drag.
I would want some kind of electrically controlled brakes on the boat trailer which is unusual. I would want a brake control in the cab set up separately for the trailer brakes so I could apply them to "straighten things out" if need be.
If your` just going 50 miles to a local lake it may work out. If you have in mind of towing it all over the place all the time I think you'll grow tired of it quickly.
A pointy, low, fiberglass hulled boat would probably tow a lot better.

Backing up two trailers is a little tricky for some people. In Alberta the lead trailer must be a 5th wheel with at least 2 axles. The longer trailer must be the lead and the overall length of the tow vehicle and trailers can not exceed 66 ft.

Don't mess with a google and use one of the rv websites that don't know the difference in towing double or triples.
ALWAYS go to your local state troop hdq and ask about legal issues regarding towing rv doubles (two trailers),

Don't do as one newby on another rv website and ask his local troop hdq if it was legal to triple tow rv's in his state (TX). Naturally he was told it wasn't. In TX its legal to tow rv doubles or double towing (two trailers) as most states.

I tow a 17' Lund boat behind my 34' 5th wheel and am 67' which is legal where I travel. Been doing this for 20 years without any issues. As far as a pontoon boat, I haven't done that but have seen many around here towing them behind various trailers, including regular travel trailers. Pontoons are heavier then many boats so that could be a factor.
If that is what you are thinking of doing then follow some of the good comments stated earlier. Have a tow vehicle that can handle the combined load, a good hitch on the lead trailer that is installed properly, weigh all units to be sure you are under all weight limits.
When traveling have a plan for stops as these type rigs are not easy to maneuver, especially at rest and fuel stops.If traveling out of your state be sure you are legal on your route. Call ahead to each state, don't rely and the internet for an accurate answer.
Above all, drive alertly and enjoy a safe trip.