I am traveling on Air France from Paris to Heathrow and then out of Heathrow on Air Canada to Boston through Halifax. Can I check my bags through in Paris or do I need to clear customs at Heathrow? I have a three-hour window for connection but am hoping I don't have to go through arrivals and customs and then re-enter for departure on Air Canada.
When you arrive in the UK from France you can indeed go through the "arrivals from the EU" lane or, if you have nothing to declare, use the green "nothing to declare" lane.
But customs usually does not take much time, it is immigration that is the bigger hold up, where passports are checked, where you need to join the queue which can be long or slow going. (And you will find immigration before customs.
It will be worth it to ask whether you can check your luggage through.
Air France and Air Canada are not in the same alliance but Air Canada has 'non Star Alliance interline partners' and Air France is mentioned in the list of possible partners.
Even with tickets bought separately sometimes airlines are able to check luggage through and you should be able to get your boarding pass by checking in online. If that does not work, ask the airline staff, Air France as well as Air Canada if needed, where to get your boarding pass.
The information about the alliance partners is on this page.