MacOS – Mac OS is reporting bad free space

To find out what takes up space, you can use Disk Inventory X, http://www.derlien.com/, which is free, or DaisyDisk, http://www.daisydiskapp.com/, also in the Mac App Store.

I would say that this is not a common task: it's actually quite complex.

The issue is that partitions are filled up from beginning to end, so the free space in your third partition is at the end of it, so it's not adjacent to your second partition.

What you need to do is to move the data from the third partition to another hard drive (or back up your entire disk to another, which would be preferred). Once that's done, delete the third partition in Disk Utility. Expand the second partition into this free space (leaving enough room for the third partition) and re-add the third partition. Finally, copy the files back to the third partition from the copy made in the first step.