Two partitions down, one left. Select the remaining free space and click on Add.
The last partition to create will be used for the root file system, and will mounted as a btrfs file system. Keep in mind that the system default is ext4, so you will have to select btrfs from the dropdown menu. The installer will also try to create this as a logical partition, but I have opted to use it as a primary partition. The mount point is, of course, /. Because only three primary partitions are created here, you are still left with one primary partitions. That is why I chose to create all three partitions as primary.
With all partitions created, click on the Install Now button to continue with the rest of the installation. That is the end.