This happened to my DD last year. The problem was that she wasn't working effectively at school. She was doing too much daydreaming when she should have been working and so the extra ended up in her take home folder. Once we sorted it out and helped DD to focus more on her tasks, the problem ended and we were down to 30 minutes a night plus reading. It was just a back to school transition thing for her.
I think a note going back to school with the homework asking for a meeting with the teacher is appropriate so that you can discuss the issue face to face. If she does indeed intend for 3 hours of homework, I'd ask to meet with her and the principal. Honestly, an average 4th grader gets home at 4:00 and goes to bed at 8:30... 3 hours is ridiculous. That only leaves an hour and a half for dinner and the rest of her life!
~ I used to be undecided, but now I'm not so sure ~ Boscoe Pertwee