I am not sure. You should perhaps contact WD.
#1 make sure that the discs are on the supported list. The model and the dash firmware must match excactly. Now this list does not list any 3tb drives for the 2 X 2 that you have, but I think they will work as 2tb drives.
MAKE A BACKUP FIRST !!! Save any important data somewhere else.
I think if you add one drive to your box it will autmaticly conevert to Raid 5 and you will have 4 TB useable. If you had the second drive later it will still be raid 5 and you will have 6tb useable. (It is only going to use 2 of the 3 TB’s on each disc. One TB is lost/wasted)
Or if you remove both of your 2tb drives and then put the 2 new 3tb drives in you can do a factory reset, then factory restore and have a 3tb mirror.
This is ONLY if the new drives are on the supported drive list. A WD RED Drive or similar will do nothing.
Evereything I have typed is just an educated guess. You should try to contact WD Directly to see if they have more info if these drives are on the supported list. If they are not on the supported list, see if you can get your money back 
