Recently, I saw manga titled [m]'Angel' with [a]'U-Jin' as author. Something's bothering me, because the information that it's stopped in vol 3. There is also comment that it's already finished in 7 volumes, I already found the pictures in Yahoo Auctions. There are 2 manga with same title and different covers, the author also the same. My only question is did that first 3 volumes of 7 is have same story that 3 stopped despite having different covers?

It's answered on wikipedia. Notes number 3. The English page. Whether it's true, who knows. I didn't check the Japanese wikipedia page.

What's with [m] and [a]? That's was weird. Is it a convention on this site? I hope you didn't just create a new convention and expect people to accept it.

The [m] and [a] in OP's post are BBCode tags that were improperly resolved, likely due to the quotes being used at the same time. No new conventions here.

I only followed the rules in FAQ to give bbcode

It's a series with the same main character

Angel (7 Volumes): https://www.mangaupdates.com/series.html?id=462

Sequel made several years after with different artstyle and storytelling:

Angel - The Women Whom Delivery Host Kosuke Atami Healed... https://www.mangaupdates.com/series.html?id=26433

5 volumes, whoever the story stopped in the middle and continues on the Season II

Angel - Season II https://www.mangaupdates.com/series.html?id=40773

I like just the first Angel