Hi Marilyn,
It depends on the type of fish. For example, a 3 1/2 ounce serving of Atlantic mackeral contains 2.6 grams of omega 3's, so 4 ounces would provide the 3000 mg you seek. However, canned tuna provides only 0.5 grams per 3 1/2 ounces, so you'd need about 21 ounces to consume 3000 mg of omega 3's.
All the best,
Lisa Nelson RD
How to Lower Cholesterol in 8 Simple Steps