Well I guess there are three possibillities:
1) they might ask the canadians to hand you over to them
2) you might have to do the service after you come home
3) or you might even get trialed when coming home
I am afraid 3. is the most likely option.
Does it say WHEN you have to do the 3 years?
Why don't you ask if the military service can be postponed for some time?
Here in Austria it was possible to postpone the military service for some time for example if you wanted to study first. That's not possibel any more but maybe it is in Israel?
Or they might just aswell make an exception if you explain them your story, I don't know. But I would definitely not try it out without knowing, I would assume you would get trialed if you just leave without telling anyone, at least that's how it is in Austria.
Only keep in mind, that it might be "dangerous" to postpone the service. Dangerouse, because think about what you can loose. I would never do that, I would finish everything (school, military service,..) (which I did) before going abroad. Imagine you find a great job in Canada, or a wife, or you come back to Israel and get a great job opportunity, or you even come back with a familiy,.... you loose everything because of the darn military service you postponed.
You know what I mean...