Since you arrived here on March 1^st, I believe you applied for your social security card after 3 weeks presuming around March 21-22 and by now I hope you already got your Social security. For the Actual green card to come, it will take around 3-4 weeks normally in most cases. Furthermore, you get welcome letter in your mail first (the address you have given to the immigration officers at the Port of Entry) then after getting welcome letter, you will get your actual green card within 1-2 weeks. You should have already got your Green card by now. So, I would suggest you to wait for a week and if not then call them back.