Solved it already by myself.
I indeed disconnected the burners and attached the two 750 GB-disks instead.
Then I switched them from IDE to RAID in the BIOS.
Next booted from the 250 GB-disks into Windows and started RaidXpert to build the mentioned RAID1 L3.
After that was finished I tried to clone the one logical drive (L1) to the other (L3) by means of using Acronis, but somehow that would not start because the destination disk
to clone to was visible but grayed out, thus not selectable.
Then I tried Clonedisk and that did the job.
I only needed to enlarge the largest of the 3 cloned partitions (almost 250 GB) to nearly 750 GB afterwards.
Mission accomplished, me happy !