At only 3 weeks into pregnancy there's no way to tell for sure that they're pregnant. If she's obviously showing, she's much further along than 3 weeks.

It's highly unlikely that she's dog aggressive because she's pregnant. But if she's dog aggressive now, she'll be even more fiercely aggressive once she goes into labor and has puppies to care for.

If she's already showing signs of being obviously pregnant--better get the whelping box ready and keep the two dogs apart!

Good luck!