When finishing a letter, we use below expressions and last time I wrote an e-mail letter for corretion and it was written: "Yours sincerely (should be either "Yours truly" or "Sincerely")" Are there any differences among Yours sincerely, Yours truly and Sincerely? is it a just matter of formal or informal? Thank you.

From my experience, "yours sincerely" is not commonly used or is a little archaic, which is why I recommended using one of the other two options. They are all relatively high in terms of formality, so I think using either would be appropriate.