I would get sorted with the format of disk3 to XFS followed by copying the old disk3 content back to the new disk3, then with your newfound experience in manipulating data on the server I would copy the content of disk1 and disk2 to disk3 as well.
Then you would be in a better position to finish your ReiserFS transition, as you could format disk1 and disk2 to XFS since the content is now on disk3.
Only after that was done would I replace disk1, and as a freshly formatted blank XFS disk, ready to receive the content from disk4 and disk5, which would conclude your transition off of ReiserFS.
Note that I would NOT move any content in this entire operation, instead COPY. It's faster, especially when the source drive is ReiserFS, and you can compare the files on the source and destination before moving forward.
Also, for the purposes of this migration be sure to only work with /mnt/diskX paths, never stray into the /mnt/user tree.
If you really can't get a grip on doing this with mc at the console, then you can temporarily turn on disk shares and copy things over the network. If you do, be VERY careful never to interact with the regular shares while you are working with the diskX shares, and turn them off after you are done.