Could anybody explain what kind of documents are required to apply for a non-B visa?

Do I need to show the wp3 letter to the Thai embassy?

Is there some different requirements between Thai embassies in different countries?

Could anybody kindly explain me?

Normally you just need a letter from the company that is going to employ you.

Depends on where you are applying, ie what thai embassy in what country

Thanks Forkinhades.

Could you answer one more question please?

I'm a Myanmar national and want to go back to myanmar to change the visa type.

Some of my friends who had done that said we need to show the wp3 letter to the Thai embassy, Myanmar. But the company said I don't need so that the won't provide me the wp3 letter. Do you have any suggestion where is easy to change the visa type?

I do mine in England, Birmingham office, but obviously that would be too far you.

You need to telephone each thai embassy in your country to find out what 'each' one requires. (I know in the London thai embassy they require all documents), its just thais interpretations, can drive you mad sometimes, goodluck