Sorry to see that you're having this issue, @xiangyu lin.
Are you able to determine which 3 files are causing the issue?
You might see them in your Dropbox folder with a blue circling arrow icon on them.
Also, if you click on your Dropbox icon, then on your initials/profile pic/gear icon and then on "view sync issues", what do you see?
Let me know and we'll go from there.