The way I see it, if you want a good all around phone I think the D900 is the choice to get. Also if 850mhz band is important to you then D900 is the only phone out of the lot that has it.
the W850, being branded a walkman phone, is probably the best out of the three for playing music. It's got dedicated controls for play functions if you find convenience something to have. Its camera probably isn't bad, being a 2mp camera just like the w800, except there is no autofocus I think, so you'd better have a steady hand to take those pictures.
lastly, the k800 would most likely take the best pictures, since it's main focus is on the cybershot branding of the phone. 3mp camera, autofocus, and a brighter flash than the other two.
Here's what i think are the important aspects of each of the phones and where the primary/secondary focus (in terms of what you're looking for) is:
D900: Slim, Slider, 850mhz, good all around mp3 player and camera (maybe equal primary/secondary)
W850: about a finger's width thicker than D900. no 850mhz. Primary: mp3 player, Secondary: camera
K800: about same as W850 in thickness. no 850mhz. Primary: camera, Secondary: mp3 player.
This is all coming from a general point of view as I don't know how the W850 and K800 actually perform. Take it as you will, I have slightly more of a bias towards the camera than the mp3 player in a phone, but really I wouldn't mind either way. I'm really just looking for a change from the candybar style of phone (my last two were candybars, and both SE), and I couldn't find a flip phone that I really liked. I also don't like the look of the W850 that much, so I'm planning to get a D900. IMO it looks pretty sweet so that helped me make my decision a little too.