In 3.x the interface has changed and it takes some getting used to.  There are a couple of different was to do it.  But to mimic how you did it in 2.x this is what you would do:

1) Click Infrastructure

2) On the left hand side in the device tree drill down to you find the device you want to modify and click on it.

3) When you are looking at the device on the left side you will see 'Components' and one of the components will be 'File Systems'.  Click on 'File Systems'

4) In the top section of the right side will list all of the filesystems that were discovered on the device.  Click on the one you want

5) Just below the top section you will see 'Display' and this defaults to 'Graphs'.  Drop it down and choose 'Templates'

6) You should now see the 'Filesystem' template and where it is defined.  There is a button that says 'Create Local Copy'.  Click that button

7) Now you can modify the threshold for just the filesystem.

Let me know if that works or not.

--Dennis