My Oasis 3 arrived today. Of course it's not much different from the previous generation, except for the warmth setting, but it seems to me the text is a bit darker, almost on a par with the Voyage. The background is about the same as the Oasis 2, yellower than the Voyage. Page turns are noticeably faster, and its predecessor was already no slouch. My Nook seems a veritable snail in comparison.
The frontlight is quite even.
Those are initial impressions and I might revise them after further testing. I tried to capture the differences in contrast in a photo, but didn't succeed very well. It's more noticeable to the naked eye.
The Oasis 2 is on the left, the Voyage in the middle and the Oasis 3 on the right.