A fatty acid with a double bond at the third carbon from the methyl end is considered an omega-3 fatty acid.

Here's why:

* Omega (ω) designation: This refers to the position of the last carbon atom in the fatty acid chain, which is the methyl end.

* Counting from the omega end: You count the carbons backwards from the methyl end.

* Double bond location: A double bond at the third carbon from the methyl end means the double bond is three carbons away from the omega end.

Therefore, any fatty acid with a double bond at the third carbon from the methyl end is classified as an omega-3 fatty acid.