Hummm.
Towingworld.com says triple tow is 'not stated', for Ontario.
I just glanced through the highway traffic act, it does not seem to mention anything about triple towing - not even for commercial vehicles?!?
Highway act - http://www.ontario.ca/laws/statute/90h08
Towing World is a RV related website and IMO as all RV websites is a poor place to find credible input how dot enforces size and weight regulations.
Towing world is calling three vehicle combination a triple tow when in the trucking world its a double tow = two trailers.
A triple tow is three trailers which is popular around here in the summer with a pickup/5th wheel trailer/ski boat trailer with a trailer full of PWC's bringing up the rear.
Best bet is a 1-800 to your state/province size and weight folks for their input. I'm between two OHP troops which is another source for what my state does or doesn't enforce regardless of what the regulation reads.
Example my state combination for RV reads in part;
** No combination of vehicles shall consist of more than two units, except:
a. one truck and semitrailer or truck-tractor/semitrailer combination may tow one complete trailer or semitrailer, or
b. vans, suburbans, blazers or other similar types of vehicles and self-propelled recreational vehicles with a three-quarter (3/4) ton or more rated capacity, may tow a semitrailer and one complete trailer or semitrailer for recreational purposes only, provided the overall length, inclusive of the front and rear bumpers, does not exceed sixty-five (65) feet.**
My local OHP troop says Because of the wording "blazers" and "suburbans" and a "3/4 ton truck" are so vague the department at this time isn't enforcing this section till the state legislature gets the wording right.
I'm using this just a example why its best to talk with those that enforce size limit codes instead of using your own (or others) interpretations.