It's difficult because you weren't there so will probably never know for sure, and it's worth a conversation with school, but i would agree with the people who say that children (and indeed adults) often interpret any communication they don't like as shouting (you only have to look at how often OPs on here describe someone as screaming at them), and indeed that teachers sometimes need to raise their voices because classrooms of 3 year olds are loud places! Your child might not even have been the only one crying, and it sounds like it may have gone on for some time.
Also, I think some of the posters are imagining some quite weeping in the corner, which maybe it was, but equally with my son I know that he has different cries, some obviously very genuinely distressed,but others that are more angry tantrums at being told to do something he doesn't want to do. These ones are usually very loud wailing, and if he was doing that in school I'd have no problem with him being told to try and stop itp as it would be disruptive and possibly upsetting to the other children and he needs to learn other ways of dealing with not getting his own way.
Of course it's wrong if she was aggressively yelled at, but a slightly stern tone or raised voice is sometimes appropriate. And actually when there is a class full of 3 year olds unfortunately every child can't be dealt with in the ideal way at every moment.
I always find it interesting that the same posters who are horrified by ever trying to get a child to regulate their emotions also seem to expect teachers and parents to manage to speak calmly and kindly at all times and never sound exasperated or a bit snappy because they are being pulled in lots of different directions at once. Yes obviously they are adults and should be much better at controlling it, but they are also not robots and I'm not sure this idea of occasional shouting being akin to abuse is all that healthy for anyone either.