see below - Omega three fatty acid

Chemical structure of alpha-linolenic acid (ALA), an essential omega-3 fatty acid, (18:3Δ9c,12c,15c).

Although chemists count from the carbonyl carbon (blue numbering), physiologists count from the omega (ω) carbon (red numbering).

Note that from the omega end (diagram right), the first double bond appears as the third carbon-carbon bond (line segment), hence the name "omega-3"