"Every", like its partner "each", is a singular pronoun. It's singular even with a list of groups, because "every" makes you consider each member of each group individually. As such, you should always use a singular verb when "every" is part of the subject.
Every American, Australian, and Canadian speaks English
Every American, every Australian, and every Canadian speaks English.
both have correct subject/verb agreement (even if they're both factually wrong). As stated by @Davo in the comments, the form with all the "every"s simply adds emphasis.
If you want to use a plural verb form, use the plural pronuon all:
All Americans, Australians, and Canadians speak English.
All Americans, all Australians, and all Canadians speak English.