I was waiting for the UM3s to come out, but then read somewhere that the sound signature of the UM3 was going to have a little bit less bass, bring out the mids and extend the treble a little bit. I'm pretty certain that this statement came from someone at Westone if I remember correctly (but I can't find the original source, so feel free to take what I've said with a pinch of salt).
Now, I've got no doubt that the Westone 3 will be an amazing product, but until it comes out it is impossible for anyone to compare them with the UM2
However, as I've heard such great things about the UM2 and the description of it's sound signature (warm and smooth) I think it may be a better option for me, as I don't want an earphone with more forward mids and treble - the UM2, as described, sound like my perfect earphone but I repfer them to be a bit on the darker side ... so I've decided to order the UM2 and they should arrive within the next couple of days.
Put simply, if the description of the UM2 sounds like it's the kind of earphone you're looking for why wait? If you've read good stuff about the UM2 but think that maybe you want something with a bit less warmth, but a bit more treble then it sounds like it worth waiting a couple more months so that you can get the Westone 3.
I'm sure that both are or will be exceptional earphones in their own right.