How does HDR movie work?
At a guess, it will be a similar technology to HDRx from RED or Magic Lantern - 2 frames instead of one recorded for every single video frame.


Reference 1: http://www.red.com/learn/red-101/hdrx-high-dynamic-range-video
Reference 2: http://cgi.tutsplus.com/tutorials/a-simple-way-to-shoot-hdr-video-footage-using-magic-lantern--ae-20993


In Magic Lantern every frame is recorded at 2 ISO values, in the case of the RED camera, two frames are captured instead of one with different shutter speeds with the second frame "2-6 stops faster".
The selection of ISO/"stops faster" is carried out by the user in both case prior to recording.

All that has to be done then, is to blend the resulting "dual frames" into a single video frame for an HDR video.
Also, keep in mind that you do not need 3 frames for an HDR - you just want to cover a greater range than a single exposure can cover, this can be 2 frames or 200 frames. For video it is 2.