Aramis went into the scent / beauty business and Porthos became the butt of lots of bad French jokes (1) but Athos?:
He moved to Peru to grow asparagus.
Incidentally, while they are termed musketeers, I've never seen any of them armed with anything more lethal than a sword.
(1). Portuguese folk are sometimes called Portos over yonder. Google blague portugais for some less than brilliantly sophisticated humour.