Allow them to pasture and supplement with feed that contain fish meal.
Here is some good information regarding omega-3. My chickens spend all (for now) their time in the coop/run so I supplement with lots of rich greens for their treats. They liked the wheat grass I gave them before so I've started growing them in trays. My layer feed (cascade) also contains fish meal.
Put the green feed to them. Let them harvest their own or cut it for them, but green feed is the way to go.
Indeed.... Free range or pasture
However in my part of the world the green world turns to white for about 6 months of the year... I am not sure of the omega-3 values of snow is that great.....SO I supplement with Alfalfa, sprouted, wheat, oats, BOSS, Flax and fish meal....
Add me to the sprouting folk. Its so easy and they can never refuse sprouts.
I did organic scratch grains in the sandy run during the summer, but now I have proper trays. In them I do alfaalfa, brassica sprouts, and loads of wheat grass which I plan to have on rotation for regular harvest.