You want this:
find / -type f -ctime +30 -mtime +30 -atime +30 -print0 | xargs -0 -P 4 md5sum
You want the list of files to be fedinto the md5sum command. This is done with `find / | xargs md5sum`. Then you want to not have to worry about crazy characters (spaces, newlines, whatever) in filenames, so we use `-print0` for find and `-0` for xargs.