SC3900 + DJM 850-K + Tracktor is posible?

Yes you can connect your SC3900s to Traktor, fortunately with more than one way!
As you know, DJM-850 has Traktor ceritifed sound card (like DJM-900NXS), so you can use it with Traktor by just connecting its USB cable.
Now for the player part: you can connect your SC3900s to Traktor with 2 ways (from what I know):
1-Classic DVS-Timecode connection: You’ll need 2 Traktor timecode CDs (because you have 2 decks) and to connect your SC3900 to DJM-850 with sound cables (RCA, Coaxial) to use that way. Godd thing is, Traktor timecode CDs are cheap so that shouldn’t be a problem for you.
2-Via connecting SC3900’s USB cables: Download the official mapping for your SC3900 here: http://b06ba727c886717f9577-fff53f927840131da4fecbedd819996a.r74.cf2.rackcdn.com//1150/downloads/SC3900_forHybridMode_120405.zip . Install it to Traktor, connect your USB cables (DJM-850 should be connected at all times) and start using it. Of course, you should set your SC3900s for MIDI use and set Traktor to use with it but these things must be easy to do. If you have USB cables for your SC3900s, you don’t have to pay for anything to use it with Traktor this way.
I specialize on Pioneer products, sorry I can’t help you with more detail.
Hope I could help.
Cheers 😀