My son had an ultrasound done on his heart(he had a heart murmur when he was born, thankfully it's gone now) when he was 1 month old. He was awake the whole time, they didn't put him to sleep, and he didn't fall asleep either. He was as still as a one month old can be, and they were able to get the results just fine. I'm not sure about the kidneys and her being 3 years old, but if she's like my 3 year old daughter, once she gets there and they start to do the ultrasound, she may sit still if the dr. tells her to, or if she's scared. Every time I've had to take my daughter to the dr. to get her ears checked or something when she's sick, all the dr. has to do is say "ok, can I look in your ears, can you sit real still for me?" and that seems to work. I hope this helps, my situation isn't quite the same, but when my 1 month old son had one done, it was fine, and he moved around as much as a baby that small does, and they were still able to do it. Good luck and God bless!