Does anybody know if Myst 3:Exile is coming to GOG? If so, How soon?

we dont know and staff wont say, best bet is to keep your ear to the ground for snippets of into that are released or contact the rights holders directly yourself. a few of our members have hounded devs and IP holders with some success

There's a fair chance, as it has accumulated a fair amount of votes on the Wishlist and Cyan seems quite nice about releasing the games onto here.

The big problem is most likely the fact that Cyan & Ubisoft need to come to an agreement first.

We have Uru, and didn't Cyan buy the rights back? Or was that ONLY Uru?

III and IV are still in limbo, there's an answer about a Steam release on their support page from October 2013:

Q: When will Myst III and Myst IV be available on Steam?
A: In order to make this happen, we need to get the legal/licensing matters handled. All parties involved would love to see this happen!

Myst 3 was developed by Presto Studios (the guys behind the Journeyman Project series), not Cyan, and published by Ubisoft. Myst 4 was developed and published by Ubisoft.

So Ubisoft owns Myst 3 and 4, but Cyan owns the trademark to Myst, that's how I think it is.

Crosmando is absolutely correct.