Disk icons appear in the order those volumes are mounted in. You might switch the SCSI IDs to try and force a different mount order.
Or you could give the disks non-sequential names. No A,B,C or 1,2,3. Name them:
Mac HD (the startup)
Stupid (the 2nd internal)
Doghouse (the external one)
Or something else, where the mount order will not bother you.