English courses

Dear Sir or Madam, ¶
I am writing in response to the advertisement I saw for your English course in “World English GLOBA.” I am interested in learntaking one of your English courses, and I would be grateful if you could provide some further information.

You "take" courses/classes and "learn" in them.
Separate complete clauses that are joined by and/but/or with a comma.

Would it be possible to have a three -weeks course?

When using a time period as an adjective, the time period stays singular. Since the adjective "three-week" comes before the noun, you should hyphenate it.

I would also like to know how much each course costs exactly.

Secondly, your advertisement mentions that each week provides research, an essay, and a report for evaluation, andn evaluation report and that the result will be announce.sod. So, I would like to know, if the result of the student's report is not satisfactory they, could they leave or canould they continue?

An "evaluation report" is a common phrase. When "so" is used at the beginning of a sentence, it is generally followed by a comma.
For the last part, I might say, "would they have to leave or could they continue," but I am not 100% sure that is what you are trying to say.

I am very keen on group studying, and I would like to know if there is any opportunity to interact with native speakers.

These are two complete clauses so you can either use a comma and the word "and" or you could add a period after studying or a semi-colon.