Flying in from Canada with Air Canada (terminal A) and flying out to Europe with Emirates (terminal B) using separate tickets (ie not booked as a connected flight). Is three hrs enough time to make the flight?

It should be enough time for you to get your bags from baggage claim go thru customs, go up to a new airline and check in and recheck your bags then proceed thru security again. I would not do less then three hours to be on the safe side unless you hv pre clearance :)

I like what Adam shares!

Yes

Should be plenty

Yes, if you go through passport control on the Canadian side, you will walk right out of A. Up to a max 10 minute wait for the Airtrain and the ride from A to B is 5 minutes.