The Xbox plays only Xbox game discs and video-DVDs of the appropriate region. Unless you've modded it, in which case I guess it depends on the chip.
Do also note that 3ivx isn't a file format in itself, it's just a video compressor. It encodes to MPEG-4 video and can so be stored in many various container formats
like AVI, Quicktime, MP4... The most usual format that is supported by mod-chips are AVI, I have no idea on the status on the other ones (but I'm sure that Google will help you).