Body contains two types of fatty acids. One are called HDL while others are LDL. You may interpret these as highly desirable lipoproteins and least desirable lipoproteins. Least desirable may also my termed as lethally dangerous lipoproteins. Your body contains both types of fatty acids. It is the balance in ratio between these two which decides the fate of dietary fat. If there is increased HDL, fats consumption will be less leading to increased body fat stores. But when LDL levels are high fat consumption will be increased leading to increased caloric burning.

There are certain speculations regarding how omega 3 helps in losing weight.

It is supposed that fish oil increases the elasticity of walls of blood vessels and improves blood flow to muscles during exercise. This helps in increased fat loss possibly due to increased delivery of lipolytic stimuli to muscles. However this small increase in blood delivery is not sufficient to lose all extra weight.

Fish oil also has some effect on appetite control and calorie intake. It results in early satisfaction and decreased hunger. The net result of this is decreased caloric intake leading to decreased fat storage.

Certain long chain fatty acids present in omega 3 increases fatty acid oxidation and decreases synthesis of fatty acids again net result is decreases body mass.

Lastly fish oil work as cofactor for certain enzymes. Once these enzymes are activated these increase fat burning in the body. However an exercise plan is important to increase working power and capacity of such enzymes.