Another recurring gripe. If you hear an advert promising "omega 3 oils", ignore it - they don't know what they're talking about.
"Omega-3" must be read as "omega minus 3". The carbon atom next to the ester bond in an oil is the alpha carbon, the next the beta. However many there are in the chain, the last one is the omega carbon atom. "Omega-3" means there is a carbon- carbon double bond 3 atoms away from the end.
This also means that "omega-6" isn't twice as good as "omega-3"!