Which Country Will I Choose for Amazon, Because Product is Available in More Than One Coun

Hi,

I am confused about some points.

- I have US, UK, CA Amazon associates program registered. If I select any product, which country will I choose?
or can I make 3 different types videos for 3 countries? Because most of the products are available in this 3 countries.

There is a option in Youtube upload page called "Video Location" is that worth enough to put UK, US or CA for 3 different countries?
Is it working?

- Suppose someone from Canada click on my UK Amazon affiliate link, Amazon will automatically redirect him to local country site, will I get any benefits from that sale?

- or should I just stick with only 1 country?

Please let me know.

Thanks