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
It gets even stranger when I went to a few forums I found various discussions. The earlier ones said yes it can be done, but only with a commercial vehicle, and then they go on to say pickup trucks are all considered commercial vehicles. These various conversations are based on a part of the HTA that no longer says anything about it when I followed the links or compared wording.
It also seems the 'towing safely brochure' the MTO used to publish has been replaced, with an online one that no longer comments either.
Personally I have only seen one person towing two trailers in southern Ontario in all my years, and they had Quebec plates.
I give up.