There is a shift in third grade where they stop using a lot of pictures and start using words. It is "reading to learn" instead of "learning to read."
It's possible she got lost somewhere and hasn't been able to find her way. Once a child gets "lost" in school, it's hard to find their way back. She needs someone to pinpoint where she got lost and to help her fix it.
I would talk to her teacher about her schoolwork and try to pinpoint what it is she is having problems with. Then you can look into peer tutoring or getting her a professional tutor.
My daughter got lost in first grade. She couldn't read, and then instead of trying she "gave up" and started causing trouble in class. She was in the principal's office a lot and one time she even tried to run away from school! In our situation we started tutoring her over the summer after first grade (in my opinion she should have never passed first grade to begin with) and it turned into permanent homeschooling.
She had trouble with our third grade curriculum because of all the reading and reading comprehension. They really do start expecting more out of kids when they reach third grade!
I have read that every child's brain develops at a different rate, but that schools usually treat kids like they are all developing at the same rate. I have noticed that six months makes a huge difference! My daughter was born in July, and she is really working one grade level below what she technically should be. Had we held her back and started first grade a year later, she'd still be the same age as the other kids in the class but she'd have had extra time for her brain to develop. I'm pretty sure if we stuck her in public school at this point, she'd be in 4th grade, not 5th. And there's nothing wrong with that since a lot of 4th graders are 10.
My daughter had trouble learning how to tell time. I gave up and dropped it, and then picked it up 3 months later. She understood it instantly! So there really is something to children's brains "developing."
Basically what I'm saying is your daughter needs help in school (not mental help and I'm sure her "depression" has a real cause) and it may just be that third grade is moving too fast for her. Talk to a teacher, and talk to a professional tutoring service. They may be able to help your sort it out!
Good luck!