If you're having difficulty getting the front fan bracket on your Ultimaker 3 or Ultimaker 3 Extended to close, this is a quick and easy fix--no tools required.
The first thing to check is that your Print Cores are seated all the way inside the printhead--they should click into place and be seated securely when you install them.
If your Print Cores are correctly in position, and your fan bracket still doesn't close, then there's a good chance the metal plate at the bottom of your fan bracket has become slightly bent. This can happen occasionally if you the fan bracket gets handled a little more roughly than intended.
To fix, open your fan bracket and hold it by the top and bottom of the front plastic piece. Apply pressure, bending it so the bottom part comes out towards you, and the top part is going up. Then, attempt to close the bracket again. It should close securely. You can watch a demonstration of this process below: