CHARLIE SHEEN hasn't been able to LET GO of the whole situation with "Two and a Half Men" . . . even though it's now been THREE YEARS since his infamous meltdown.

On Wednesday, ASHTON KUTCHER called him out about it on "Jimmy Kimmel Live", saying, quote, "Dude, shut the [eff] up.  Like, enough already.  Like it's three years later and you're still blowing me up on Twitter.  Come on, dude!  Really?"

Well, yesterday, Charlie APOLOGIZED.  Sort of.  He Tweeted, quote, "Ashton:  Message received.  So sorry.  You sounded like me!  Well done!  My bad.  I was pissed at other crap, and took it out on you.  Hope you are good."

But then, Charlie followed it up with this:  Quote, "But news flash:  Dude, you ever tell me to shut the [eff] up, EVER again, and I'll put you on a hospital food diet for a year.  Ya feel me, Jr.?"