Most of the difference you're seeing is due to Time Machine's "Local Snapshots" feature. When Time Machine is turned on but the backup device isn't available, it backs up to the local volume. The space used for these local snapshots is counted as "in use" by Disk Utility and System Information, but not the Finder (see the Disk Space considerations section of this article). While the space for backup actually is in use, it'll be freed automatically when needed (i.e. when the volume gets above 80% full), so the Finder counts it as being available.
In your particular case, the space System Information lists as being used for "Backups", 98.36GB, is very close to the difference in free space listed by the Finder vs. the other two. The Finder's 206.43GB free - 98.36GB of backups = 108.07GB actually free; compare to System Info's listing of 108.44GB free and Disk Util's 108.31GB. I'm not sure what the rest of the difference is (maybe they looked at the disk at slightly different times? Or they may be counting volume structures a bit differently?), but it's very small.
Finder shows:
- easily, the truth for JHFS+ volume Macintosh HD
- optionally, the truth for mtmfs volume MobileBackups
Hint: with local snapshots enabled in Time Machine, in Finder you can go to /Volumes/MobileBackups then get info.
Results of a one-line command show that for some purposes, MobileBackups is treated as a distant file system:
qlmanage -m disks | grep MobileBackups && mount | grep MobileBackups
As Disk Utility is oriented to local file systems, we'll probably never see MobileBackups as a separate volume in that context. There are degrees of simplification in Disk Utility, even when debug options are chosen.
Check the Storage section of About This Mac. You will probably see the missing 15GB listed there as "Backups". These are Time Machine local snapshots, and Finder ignores them when calculating free disk space, because they get deleted automatically when your free disk space drops below 20%. In other words, they give up their space automatically as needed, so the number in Finder is the one you should pay attention to.
Best Answer
If you have Time Machine Local Snapshots enabled (and I'm fairly sure it's enabled by default), then it's going to use some of your free space for creating these local snapshots. However, Finder doesn't take the space used by this feature into account, pretending it is actually free, because if the user starts filling up the disk, then the local snapshots get deleted -- it will only use `idle' free space, so to speak. See this other question for more details.
So, don't worry, if you actually need the extra space reported by Finder, then it will be made available to you when the need arises.