I can't speak for those fighting for the British or French directly, but estimates put some 35-40,000 Americans fighting with the Canadian Expeditionary Force (CEF). Many, if not most of these men, had some degree of ties to Canada, either through dual citizenship, or family from there. The number also includes those who may have technically been born in America (and therefore birthright citizens) but lived and resided in Canada at the time of their enlistment.

This site: http://www.americansatwarinforeignforces.com/named-americans-in-the-american-legion-cef.html has images of the pages from the four "American Legion" Battalions serving in the CEF and lists enlisted soldiers and officers. We can see that many of them have previous experience in the US military, and list their next of kin as being in the US. We can also see that most coming from America came from areas geographically closer to Canada, which is not surprising (Although I did spot a couple boys from Georgia and one from North Carolina)

By enlisting in Canada's military, they were swearing allegiance to a different nation, which could cost them American citizenship. Certainly many would have simply lied about their nationality, and its unlikely Canadian officials cared to press the matter too much in war time. So the number Canada initially presented of 35,000 serving is almost certainly a bit low, but would be impossible to determine exactly how many served from the US.

At least three Americans received the Victoria Cross, and their individual wiki pages can be found here here and here. All three served in Canadian units.