In a scientific article, should I write "3m", "3 m", "3 meters", or "3 [meters]"?
This guide at the NIST site is a good place to start.
In particular, you should write "3 m" as in "The bar is 3 m long."
The abbreviation for meter is m; it is not written between brackets, and you need to add a space between the value and the measuring unit.
The absence of brackets, and the presence of a space to separate the measuring unit from the numeric value is not limited to meter, but it is common for all the measuring units.
Quoting from Wikipedia:
The value of a quantity is written as a number followed by a space (representing a multiplication sign) and a unit symbol; e.g., "2.21 kg", "7.3×102 m2", "22 K".
This rule explicitly includes the percent sign (%). Exceptions are the symbols for plane angular degrees, minutes and seconds (°, ′ and ″), which are placed immediately after the number with no intervening space.
So three meters should be written as 3 m.