They all seem fine to me (though I know native speakers have their own preferences with language and will often scream "No, you can't say this!" without realizing they are just taking their own prejudices as fact, prejudice being their own interior language vocabulary/usage.).
will seems to mean that the person will return quite a long time in the future.
Going to seems to mean that the person will be coming back in the near future.
Only real difference and shows the thought of the speaker asking the question.
hi native speakers,
When will you come back to Canada?
When are you going back to Canada?
When will you be coming back to canada?
In sentences 1 and 3, the speaker is in Canada.
#1 means 'When will you arrive in Canada?'
#3 can mean the same as #1, but can also be used when your trip back to Canada will extend over a long period of time. For example, your trip back to Canada may involve three days of travel on a ship.
In sentence #2, the speaker is not in Canada, and the focus is on when you will start your trip back to Canada. Your trip back to Canada may take a very short time or a longer time.