¿Cuál es la diferencia entre However, my uncle refuses to quit smoking. y My uncle, however, refuses to quit smoking. y My uncle refuses to quit smoking, however. ?Puedes dar oraciones como ejemplo.

in terms of meaning? nothing
the different phrasings would come into play in a narrative, because each way flows slightly differently. one phrasing might work better in a story format than another, whether because of the phrasings in the sentences surrounding it or because the author wants to emphasize something.

the first and the last phrasings contrast the entire sentence. perhaps the sentence before discussed a frequent argument in which the author tries to persuade their uncle to stop smoking. the placement of "however" would depend on flow.

"My uncle, however, refuses to quit smoking" contrasts "uncle." perhaps the author's aunt quit, but the uncle refused