Back of the a3000 has a 25 pin scsi connector.
all you need to do is buy an external scsi box and a cdrom and make sure you get a scsi cable that has the scsi pin connector same as the box on one end and a 25 pin on the other end.
box similar to this
ebaythe cable with this looks like 50 pin for box end and centronics type for computer end, so you would need to change it, probs a couple of pound from a computer shop.
this is probs the cable.
ebayyou would have to confirm with cdrom seller which connector is on the back of the drive.
hope this helps.