'nothing can be on 3' means that the controller is on 3. The sampler has a controller and a microprocessor- essentially, a computer, albiet a special purpose one. The ASR-10 only uses Fast SCSI, so it can only use SCSI IDs 0 through 7, any ID higher than that can make the chain act odd.
Also, make sure termination is OFF on the second to last device externally, and ON on the last device in the chain.
And people wonder why i'm glad to not have to deal with SCSI anymore.