I'm sure Aramis at least bathes regularly. He's got a vested interest in keeping his hair/teeth/skin/etc. pretty. And Porthos cares deeply about appearances and the importance of nice clothing--not a man to go too long without donning a fresh shirt.

Yes! Whereas Athos probably doesn't care how he looks but would scrub down and change his underwear regularly without thinking about it because that's proper, and d'Artagnan would do it because Athos does it which must mean it's the thing to do.