3 and 5? no difference
driver and passenger? yes there is a difference and different part numbers
yes it needs the bolts and nuts, the seats just have a hole where its meant to attach to and you won't be able to do so without the speed nut and bolt
For the A3 it lists that the under seat storage is 1 part number for both sides...
8P0881581 4PK - Soul Black - around £45 each