Sounds simple enough, and for most people it will be - just watch a total of 3 hours of content. This can be gained over multiple movies, shows, etc.
However, if for whatever reason your streamed content gets interrupted and you get a "something went wrong" message (or whatever it says) you may find any time from your session spent watching that content will be lost.
For example, when I watched my first 2 movies this happened several times and I was unable to pick up from the point they stopped streaming - I had to start from the beginning and fast forward each time. Both movies totalled over 3 hours in length but this was not reflected in my progress when I finished watching them. Only the time spent from my last starting point to the end of each movie was counted. Once I was able to watch another film that was not interrupted by the service dropping out the 'cheevo' popped.
This can also affect the following achievement;