In a scientific article, should I write "3m", "3 m", "3 meters", or "3 [meters]"?
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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:
So three meters should be written as 3 m.