If you mean RAID as in raid0 or raid5, then you can set it up with 3x 2TB disks. You can include the 6TB but you'll loose 4 TB.
In Windows, you have another option - I think it's called JBOD - that just append all disks into one big, but no performance boost (no striped raid at all) and neither any safety level (you loose all files if one disk go dead).
If you're using Linux, there are a better option, the filesystem BTRF that has built-in support for raid (raid 5 is not well supported yet, but you can use LVM instead for that). And it have a much better solution for appending the disks together as one big volume:
- The metatada is cloned between disks (this is optional), so if one disk die, only files physically located on that disk are lost.
- You don't have to use the whole 6TG for that. You can partition the drive and install the OS and programs to another partition on the 6TB drive that is not part of the big volume.