It might be a little longer than 3 weeks until the next litter. It's thought that when dwarf hamsters get pregnant while already nursing a litter they can delay implantation such that the pregnancy is extended. This is an adaptation that allows them to have back-to-back litters (though in captivity it's still not a good idea of course). If you don't have more pups when the current litter is 21 days old, I wouldn't assume the mother isn't still pregnant.
https://www.ncbi.nlm.nih.gov/pubmed/9143142