why is my system data three times the size of my own data

The Storage tab is known to report incorrectly. Also check by choosing your Mac HD and "Get Info" (command-i). It may be different. Try rebuilding Spotlight index.

How to rebuild the Spotlight index on your Mac - Apple Support

Some of the space may also be from Time Machine Snapshots.

About Time Machine local snapshots - Apple Support

After you've freed up some space download Omni DiskSweeper:

https://www.omnigroup.com/more

DiskSweeper can give you a more accurate read of disk space than Finder or Storage Tab of About this Mac. It can also show you the precise size and location of all your files. It will inventory your disk starting from the files that take up the most space. If you want you can even delete files from OmniDisk Sweeper. But be careful some of the items may be important to your system.

Check Mail menu > Window > Connection Doctor. If you have check marked “Log connection activity” turn it off.

For information about the Other category in the Storage display, please see Apple Support Topic

https://support.apple.com/en-us/HT202867

Purgeable Space:

If you are using iCloud Drive those files are marked as Purgeable on your Mac, which means when you run low on disk space they will be overwritten.

Also, empty the Trash if you haven't already done so. If you use iPhoto, empty its internal Trash first:

iPhoto ▹ Empty Trash

In Photos: File ▹ Show Recently Deleted ▹ Delete All 

I never trust those storage graphs.

Boot into Safe Boot mode, and then reboot normally. This may reclaim some System consumed storage in caches, etc. It will also run a check of your boot drive.

1st off ignore the report in Storage & the categories there are meaningless.

What OS are you on?

Purging local backups

Please note that although this doesn't affect your remote backup from Time Machine, this will get rid of the redundancy (at least until the next Time Machine backup) that a local backup disk will provide. If you need such redundancy or are worried about the recovery of your data then you would be best served to let macOS determine when to purge these files.

Start Terminal from spotlight.

At the terminal type tmutil listlocalsnapshotdates.

Hit enter.

Here, you'll now see a list of all of the locally stored Time Machine backup snapshots stored on your disk.

Next you can remove the snapshots based on their date. I prefer to delete them one at at time. Once my "System" disk usage is at an acceptable level, I stop deleting but you can delete all of them if you want to reclaim all of the disk space.

Back at the terminal, type tmutil deletelocalsnapshots YYYY-MM-DD-HHMMSS , where will be one of the dates from your backup. This will be in the form of xxx-yy-zz-abcdef. Try to start with the oldest snapshot.

Hit enter.

Repeat for as many snapshot dates as required

http://www.thagomizer.com/blog/2018/03/27/cleaning-up-time-machine-local-snapshots.html

Oh, I think you missed a space, or I did...

tmutil listlocalsnapshots /

After listlocalsnapshots & before /

https://ppolyzos.com/2017/10/20/how-to-manually-delete-local-snapshots-from-time-machines-backups/

If you want to delete local snapshots manually, just turn off Time Machine and give it a few minutes to automatically delete them. When you turn Time Machine back on, it remembers your previous backup disks.

Time Machine saves one snapshot of your startup disk approximately every hour, and keeps it for 24 hours. It keeps an additional snapshot of your last successful Time Machine backup until space is needed. And in High Sierra or later, another snapshot is saved before installing any macOS update.

https://support.apple.com/en-us/HT204015

At least the command worked, just no local backups is all.