I would not mail a second copy at this time. if the IRS did receive the first copy, sending a second copy could further delay processing. wait the 3 weeks. if you see nothing call them. note that returns that are mailed in have to be manually processed. with possibly thousands of them, you should realized that it takes time for this
as Texas Roger stated - any time you mail in a return make sure you get some form of proof of delivery. this is just as important for original returns as amended. you may need that receipt to prove timely filing.