3 week old puppy question

My sister's latest rescue turned out to be pregnant, and many medical challenges later, the surviving puppies are three weeks old today. I'll copy an excerpt from her message to me about one of the puppies:
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Last night when I was helping Butter Bean get around, I bumped the box with my foot. Nilla was sleeping, and she instantly let out a growl!! I put my hand in on her and she jumped and turned quickly - if she was more coordinated (and had teeth), I think she would have snapped at me! And she is three weeks old - we better do something right away... but what?___

Butter Bean is the mother (just gone lame last night) and Nilla is the puppy. Do you wonderful experts think there is cause for concern, and have any recommendations for helping this puppy grow up well? My starting suggestion is even-more-than-usual exposure to new things for this puppy so that she develops the ability to deal with new and scary things. In her three weeks of life she's shown herself to be a very assertive little puppy, and while her father is a complete mystery, it looks like she may grown up to be a big girl. When an acupuncturist came to the house about a week ago to treat an ailing puppy, Nilla stared at her with a very alert, almost challenging 'we don't know you' expression.

Thank you for any thoughts!
Anita