Dentition class 3 doesn't say anything.  There's skeletal and dental divisions...  Maybe your mandible is skeletally protrusive?  The facial aesthetics ideally are in balance.

So called "Mewing" in its forms can actually shorten the face... bringing the lower jaw more forward.  I have no evidence of it helping a class 3 dentition.  Quite the opposite... keeping the teeth closer together drives the mandible forward... and shortens the maxilla...  so maybe it's good that it hasn't worked so far.

Determining your current nasolabial angle may be one key piece of information to look at.  Determining the proper maxillary position can be done through a number of metrics.  Dr. Derek Mahony recently produced a video on this:  https://www.youtube.com/watch?v=CAsBdrZmFmk

Sorry but I don't understand what you're saying ? 

You'll understand more by watching these videos:

https://www.youtube.com/watch?v=AMGtcYXp95Q

Skeletal and dental divisions can differ.  How do you know that your maxilla needs to be advanced?  Maybe your chin needs to go back?  I don't know.  There are skeletal and dental placements.  The only reason I'd listen to an orthodontist on not advancing the maxilla is if my nasolabel angle was good already, among other things.  I cannot comment personally on your situation... sorry!

I will say this... listen to this audio: 

https://www.orthotown.com/blog/post/9121/episode-21-dr-john-graham

23:18 - 28:42

Also... look at  http://www.airwaydevelopment.com/2014/06/link-airways-obstructive-sleep-apnea-faces/ on classes & airway

Also... whereabout are you living?  I may throw a recommendation your way.  I recommend any orthodontist at  https://www.orthoscience.com/team... I actually did make a post on this...

/community/community/main-forum/3d-cbct-airway-orthodontists/#post-10386    I'd consult with any of these.  Personally, I would be wary of pushing back anything (maxilla or mandible) without knowing my airway.  In the interview with a former oral and maxillofacial surgeon turned orthodontist... Dr. Graham... he speaks on the airway as being #1 and I concur.  He is VERY good with CBCT in office.  So... if you're close to Salt Lake City, UT or a JetBlue airport in U.S.. I'd go to him for consult.  I've been eye witness to a gorgeous job he did with a class 3 girl who others said would need surgery.  Now... she's a little girl... so may be different with adults.  I like relying on an orthodontist who has a history as a surgeon... though few exist & usually orthodontists have surgeons who they consult with and pass to or work along with.  Such as the video below.  Others on this list also employ CBCT... so they won't just try and align things while messing your airway up.  They won't.  Can't say the same for other orthodontists.  I can say, though, that even a cephalometric will show airway... that CBCT isn't the only way to see airway.  The overall volume and MCA (minimum cross-sectional area) can be known with certain software of CBCT.

Lastly... watch https://youtu.be/yFPwOM8Azz c"> https://youtu.be/yFPwOM8Azzc

However... this is all just to educate.  I think the more educated the better.  However, it's really up to you on what you want to do.  Whatever you do...  Just to be informed and to know what to do.  If any of these who use CBCT say to move back the chin, I'd do it... that's me personally.  Because, they're airway aware.