It depends on the video format. For example, 1080p H.264 content is no problem and can be played natively using QuickTime. MKV should pose no problem either, unless the content is hi10p. For a more detailed explanation, refer to this post by Menneisyys2: https://forums.macrumors.com/threads/1547504/.
DTS support as of this writing is NOT included in nPlayer or any other player available in the App Store currently. This may or may not change, I've heard that some developers are negotiating the license for DTS support, so keep your eyes open.
I have no info on 3D other than it's not supported by any player in the App Store as of this writing. I don't see it changing either as the iPad lacks an autostereoscopic display and anaglyph 3D is simply a mess.
I could do OK with anaglyph 3D on the iPad. Keep in mind that it can be connected to a TV, monitor or projector. I don't expect more than that at this time.