If there was any merit to moving files in parallel, there's a good chance
mv would already be doing that by itself, or a similar tool would exist. ;)
There shouldn't be any point in running
mv in parallel on the same source/target disk, since you're usually limited by storage media speeds anyhow. If
/usb_storage/ is a USB stick, it won't make the stick go any faster.
mv /db/dbfile1 /db/dbfile2 /db/dbfile3 /usb_storage/
Should be the best method since it does what you want, blocks until it's finished (for your alternative to be roughly equivalent, you'd have to add a
wait at the end). Writing files sequentially instead of in parallel also helps to prevent unnecessary file fragmentation.
Also, in case
/usb_storage/ isn't large enough to hold all the files you're moving onto it, instead of ending up with three incomplete / no copies when working in parallel, you might get at least one complete file with the sequential solution.