question about RAID 0 total capacity

Ok i posted this question a couple of days ago. i'm sure there's someone out there who is a lot smarter than me who has an answer for this??

i have three internal HD's on my computer which are as follows:

OWC_SSD - 1TB,
Apple_HFS - 2TB, Apple_HFS-1TB

i wanted to combine the two Apple HD's in RAID 0 to make a single HD of 3TB. Since RAID is no longer available in DU I attempted to create it through the terminal.

i'm very much a novice in using the terminal but found the correct command online and it produced the RAID Set but the total capacity is only 2TB?

Does anyone know why this happened? Also would this be the command to take the two HD's back to their original state?

diskutil destroyRAID /Volumes/StrippedVolume if i needed to?

thank you