between is used for two items..
among is used for more than two item..

between X and Y
among X, Y and Z

simplyjat wrote:between is used for two items..
among is used for more than two item..

between X and Y
among X, Y and Z

Correct!

We'd using within in the context of within a group or geographical area.

For example:

"Among Canadians, moose are the most popular pets."

"Within Canada, moose are the most popular pets."

See if this also helps:

From Sumit's notes:

Among v/s Between: The simple rule will rarely fail you: use ‘between’ for two things, among for more than two, but that generalization does not describe the way English has long used these prepositions. ‘Between’ can be used of as many items as you like if the relationship is one-to-one, however much it may be repeated with different partners: Economic relations between Great Britain, France, and Italy [or between some members of the EEC] are tense at present.
‘Among’ works with any plural number above two. When we're talking about a one-to-one relationship between two groups, ‘between’ is correct. Now imagine we still have those relationships, but multiplied. For example, diplomatic relations between countries or friendships between people.
Notice how the meaning would change for these:
• Friendships among people. (Sounds like it could be a three-way friendship.)
• Friendships between people.
• Relations between countries.
• Relations among countries. (Three-way (or more) again.)

Thank you Stuart and Netigen for your answers. I knew the difference between "Between" and "Among" but I was not sure about "Within".

Thanks,
Franco