Hiya,
I didnt want to read and run. I suggest you visit your GP and request bloods if you think it may be a new pregnancy. However if it was just 3 weeks ago you had a mmc i think it would be too early to show up on a test at the moment so its more than likely left over HCG.
There is hope though, i conceived my daughter just under 2 weeks after a mmc so it is possible! Good luck and i hope you get some answers xx