Can Omega 3 Fatty Acids prevent chest pain in periodic Atrial Fibrillation?
Omega 3 Fatty Acids cannot prevent the the acute symptoms of periodic A Fib, nor is there a role for vitamin D in the treatment of acute symptoms of periodic AF.

Atrial Fibrillation with the symptom of chest pain is very worrisome and requires urgent or emergent treatment. The most common treatments are heart rate control via IV antiarrhythmics, or cardioversion, either electrical or chemical, usually done in the emergency room. 

There has never been a study wherein AFib accompanied by symptoms of chest pain was treated by Omega 3 Polyunsaturated Fatty Acids and there never will be, because N3PUFA does not cardiovert arrhythmias nor does it effect heart rate in AFib.

No medical body of any kind would support such a study. 

Finally, there is no convincing evidence that N3PUFAs decrease the incidence of AFib, and this has been studied often, because eating 4-5 servings of fish per week has been shown to have a beneficial effect on A Fib. That gives reason to hope that N3PUFA supplements might be similarly beneficial. However:


  The role of omega-3 polyunsaturated fatty acids in the setting of AF is controversial. Although some studies demonstrate a lower incidence of AF recurrence with omega-3 polyunsaturated fatty acids use, others have shown an increased risk. Further studies are needed to assess the role of N3-PUFAs in the prevention of AF.[1]


If N3PUFAs have not been proven to help AFib long term, there is no hope whatsoever that studies will be undertaken to shown their effect in the acute treatment of symptomatic AFib, which is the scenario you have described.

Atrial Fibrillation becomes symptomatic when the arrhythmia results in decreased perfusion of various organs. If there is decreased perfusion to the heart, the presenting symptoms may be of ischemia (usually chest pain or pressure) or shortness of breath due to decreased cardiac output and decreased oxygenation in the lungs. If it is severe enough, fainting may occur from decreased blood pressure, and decreased perfusion of the brain. If it is not severe, it may result in a sensation of flip-flopping in the chest, or palpitations (irregular, thumping, or pounding heartbeats). There are other symptoms of A Fib but since you mentioned chest pain, I won't elaborate.

[1] Atrial Fibrillation in the 21st Century: A Current Understanding of Risk Factors and Primary Prevention Strategies