Disk Utility in High Sierra 10.13.1 partitioning fail
You have to remove the recovery partition - it's probably broken; at least it's consuming 80% of your disk. Then you have to resize your main partition disk0s2. Finally restore/rebuild the recovery partition.


Open Terminal.app and enter diskutil list to get the details
Check the disk: diskutil verifyDisk disk0.
diskutil eraseVolume Free n disk0s3
diskutil resizeVolume disk0s2 R to resize the main volume to the max.

If the R size option fails try to use a real size like 120g or the percentage 100% (on some external disks this may fail because of some 2 TiB limit).

diskutil resizeVolume disk0s2 1000g


or a little less if this also fails

diskutil resizeVolume disk0s2 999800m

Recreate the recovery partition by downloading and reinstalling the latest High Sierra version (10.3.2 as of time of writing). Reinstalling won't delete your data. A Time Machine backup is recommended though.