with PCE S³ is it possible to transfert old save data from interface unit/duo with ten no koe card ?
I haven't tried, but it's theoretically possible, as when when a real hucard is detected, the backup ram controller is still enabled, and you can read/write it, and it will be saved to the sd.
The process, though, will be a bit involved, depending if you have "per game saves" selected or not.
If you don't have the per game saves selected, the save data will be read and written to a file named backup.bup, so you have all the saves in a block, as if you had a normal super cdrom/IFU. Just insert your ten no koe card and transfer the saves to the cd backup ram.
With the "per game saves" option on is a bit more difficult, because each game will have its own .bup file, but when running an external hucard, it will use backup.bup, as it doesn't know which game you have, so you'll have to insert your ten no koe card, transfer the data from the card to the cd backup ram (of the SD system 3) then take the sd out, go to the bup folder and copy or rename the backup.bup file to the name of the game you want to use that backup for, and then you can run your game with the save data you transferred from the card, although it will have more save files there, apart from the current game, you can delete them from the backup ram manager in the bios (if you have "Skip 'Press Run' " on, just hold SELECT while booting, and it will not skip the press run message, and you can access the backup manager by pressing SELECT there.
Edit: This is untested though, it's just that if it works, it will work that way.