Is this a trick question?
In Raid 5 you lose 1 disk worth of capacity. So 3x2TB = 6TB, but lose 2TB capacity = 4TB.
You also lose some logical space due to formatting overhead etc - but should have 3.7TB or so available.
Adding 1 more disk will increase it to 6TB, leaving around 5.7TB usable.
(NOTE: To add another disk, you will need to perform a migration, which WILL expose your data to risk, and you may need to purchase a Battery/Capacitor module if your raid controller doesn't already have one fitted)
Just to bear in mind though - RAID5 for most things is a bad idea. Ideally you should migrate it to at the least RAID6 (to prevent a double drive failure causing data loss), or if possible RAID1 / RAID 10.