Thank you for asking the query :
At three weeks of pregnancy you need to take only Folic Acid as supplementation. Folic acid 0.4 to 0.8 mg is the recommended doses. Higher doses like 5mg are also prescribed by certain Doctors which are also acceptable. Folate deficiency can cause neural tube defects and folic acid supplementation can prevent them. Periconceptional folic acid supplementation reduces the incidence of neural tube defects by 50 to 70 percent. Take nutritious food. There could be nausea and vomiting in the first trimester, but this will soon disappear. Please take care to drink plenty of water and stay adequately hydrated. Healthy women with uncomplicated pregnancies should continue to exercise during pregnancy. From second trimester you make take Iron supplements also. Please note this recommendation is based on the assumption that you are a well nourished person with no medical illness or complication. If this is not the case please let me know.
Hope this helps. Please feel free to ask followup questions via this email itself.