I've had two litters of pups now and both times I didn't think the moms took. They didn't show until the last week of pregnancy (21 days till birth), but they only look chubby. At 3 weeks your moms should look prego, but they might not have taken the first week. Both of mine were exactly 1 day late from the day I exposed them to the male. They look chubby a week before and a day or two before they give birth they look HUGE.

Your females might also only have a few babies, if that's the case they might not even look pregnant.

Also, if you are wanting breeding females you'll want them to be under 10 months old. They usually stop growing around 6-8 months so you can judge their age by that. I'm not an expert, I'm just regurgitating what I've read.