In-engine just means the scene was made using the developed engine for the game... there is no gameplay logic, the hardware can be a TOP PC, a PC target console hardware, actual final console hardware, you can render in what framerate you want and speed up to match the desired framerate (like pre-rendered).
Real-time is when a scene is rendered by gameplay running on the final hardware with.
Pre-rendered... the best you can do on TOP PC hardware (or PC farm, cluster, etc) that have no limitations of the game engine... it can take days to render a frame after it is speed up to mach the desired framerate.
The biggest difference between the three are:
- Pre-rendered: No game engine limitation... you can reach the heavens
- In-engine: you can do anything your game engine limitations allow without gameplay.
- Real-time: what you actually see in gameplay.
This is Uncharted 4 in-engine vs actual gameplay trailer (the first part of the video after they are comparing with 2015 trailer that was already inferior to in-engine 2014 one):