When it comes to large desktop cases, there are usually several 5.25-inch bays available, most of which stay unused forever, since a single optical disk drive is enough for everyone.
For enclosures with at least two such bays free, Icy Dock has released the FlexCage MB973SP-1B
The company describes it as a 3-in-2 Tray-less SATA Backplane Cage for 3.5" SATA hard drives.
In other words, it is a drive cage that occupies the space of two 5.25-inch drive bays but can hold three 3.5-inch hard disk drives.
Tool-less drive installation is implemented, as is Active Power Technology (the LED and fan only activate when a drive is present).
The FlexCage gets energy via two 4-pin MOLEX connectors and has individual SATA drives (SATA 6.0 Gbps supported) for each storage unit.