You can review the site, below, and learn more about the basics of editing film.
Every director choose the primary techniques that fit the story s/he is telling.
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Temporal
spatial
Non-Linear editing
If you are asking about 3 pic or picture editing that is the merging or three pictures into one. There are many photo editing programs that can help with overlay of photo, shadowing and editing.
Seamless editing is where you seem to know what you are doing with your editing, but the more and more you get deeply involved with the project you get less aware of what is happening.