I'm just a user, so I'll let the Amara people reply in detail on
formal team structures that can be used. Meanwhile you might wish
to have a look at / join one of the teams listed in http://blog.universalsubtitles.org/volunteer/
From my user's experience, though, it seems to me that you might
not need a formal team structure as you all already collaborate and
communicate. The problem with a 3 hour video is the transcribing.
Could you consider splitting it into shorter parts, on the final
Otherwise, if you want to have the video in one block, you might
agree on a few simple rules, like always marking the timing you've
reached when you stop transcribing, and possibly also for passages
you don't understand, so that the others can easily find these
points before the transcript gets synchronized into subtitles. When
you synchronize the transcript into subtitles, you can then remove
these added markings.
Where were you thinking of hosting the video/s?