Compose means to form by putting together. In other words, the sum is composed of its parts. Think about composing music, which is basically putting musical notes together.
Example: Canada is composed of 10 provinces and three territories.
Comprise means to be made up of, or to include. “Comprised of” is redundant; it is better to use “composed of” in such a sentence construction, as in the example above.
Example: Canada comprises 10 provinces and three territories.
Constitute means to be the elements of, to make up, or to form. Think of all the elements of government that make up a formal Constitution document. Constitute may work best when neither compose nor comprise seem to fit.
Example: Ten provinces and three territories constitute the country of Canada.
(Note that you spell out the number at the beginning of a sentence.)
Fun Fact: The Canadian Constitution was finalized in 1982, when the British and Canadian parliaments passed parallel acts: the Canada Act 1982 in the UK, and the Constitution Act 1982 in Canada. Thereafter, the UK was formally absolved of any remaining responsibility for or jurisdiction over Canada. In a formal ceremony on Parliament Hill in Ottawa on April 17, 1982, England’s Queen Elizabeth II signed both acts into law.