Try this configuration:
1) Plug the UniDisk 3.5-inch directly into the back of the computer (no
controller).
2) Plug the DuoDisk into the port on the UniDisk 3.5 disk (no controller).
3) Plug the single 5.25-inch UniDisk into a controller card in slot 7.
Set the Control Panel as follows:
slot 5 smart port
slot 6 disk port
slot 7 your card
startup scan
This configuration tries first to start up from the UniDisk 5.25. If no
disk is found, then it looks to drive 1 of the DuoDisk. If no disk is
found, then it goes to the UniDisk 3.5.