Honestly If we want to play 3 at the back, we should lineup and attack in a similar way to Belgium. I really like Tifo footballs video on their tactics:
TL;DR: Belgium lineup with a 343 using a flowing front three. Belgium also play their key playmaker ,KDB, as part of the Midfield two alongside Axel Witsel.
That being said Canada's 343 would look more like this :
CBs: Cornelius, Hutch/Vitoria/Ferriera, Henry/James
Ideally that front three is fluid but with David just behind Cavallini and Hoilett being able to provide with in either side. Defensively, Canada won't be able to high press like Belgium as we can't rely on Cornelius and Henry the way they do for Vertonghen and Alderweireld. What Canada can do in this case is have the front thee press high while the rest of the team drops back. This is why I would have Hoilett over an extra midfielder, His pace and dynamic play is something this this team needs more then a body in Midfield. Davies can get forward in a way similar to Chadli or Carrasco.