Particle DE (で)
The particle で (DE) is used to mark a location where an action takes place. It's important to understand the difference with に (NI), which is used simply to mark a location with no specific action taking place.
で (DE) can be used to describe the material used to make something. It means "made of, etc." Make the subtle difference with "made from" (raw material) for which から (KARA) should be used.
The particle で (DE) can express the means of doing something or going somewhere. A good translation is "with" (with a pen), or "by" (by train). Don't make any confusion when trying to translate "with someone". A person is not a means of doing something, but means "together with someone" in Japanese. In that case, use と一緒に (TO ISSHO NI).
Note that the means / the cause can have a negative result, too. で is in fact also used to express the means by which something negative occurs. In that case, で can be thought as a way to explain a reason.
The particle で (DE) can be used to express the length or scope. This is how to translate "within" for example.
The Japanese sentence keeps the idea of a means: "With 3 hours, I could finish the game!"
In a superlative structure (the most / the less + adjective), で (DE) is used to indicate the group in which the elements are compared.
で (DE) marks the moment when something is expected to occur.