Have you looked inside the mouths of your other two Meller's?
If I remember correctly...the inside of a Meller's mouth is
always/usually black.
Do you have a place where the chameleon can lay eggs if its a
female?
Are you keeping the three (or even two) in the same cage?
If its not eating, then and keeping its eyes shut...then I would
be taking it to the vets. Its not normal to be sitting with its
eyes closed all the time.
You may be keeping the upper temperature a little high......
http://ADCHAM.com/html/taxonomy/species/chmelleri.html
Not a very good picture...but it shows a little of the
inside of the mouth...
http://ADCHAM.com/images/images-species/Ch.Melleri/ChamaeleoMelleri1.jpg
Here's a couple of articles that (although I don't think give
you the answer to your problem) you might like to read...
http://www.chameleonnews.com/year2002/nov2002/melleriprofile/melleri_profile.html
http://lllreptile.com/info/library/animal-care-sheets/chameleons/-/mellers-chameleon/
http://www.melleridiscovery.com/
Mellers are not one of the easier chameleons to keep in
captivity...especially when they are wild caught.