AMD has revealed the die shot of the cIOD for its Zen 4 processors in its recent ISSCC 2023 presentation. Thanks to annotations by @Locuza_, we now know for certain that it's not possible to use the current I/O die with three CCDs.
The cIOD has two GMI3 interfaces, to which the CCDs are connected. The 2x 40-bit memory interface is for ECC memory support outside of the on-die ECC support of DDR5 memory. Zen 4 CPUs support ECC memory, but it's up to the motherboard makers to implement it.
Even a basic GPU takes up a considerable amount of space inside the cIOD, which already has parts related to video decoders/encoders. These parts take up at least a third of the space inside the I/O die. The die has undergone significant shrinkage from the Zen 3 era cIOD, while having an estimated 58% increase in transistors.
AMD Zen 4 I/O Die Revealed: Detailed Annotations and Die Shot from ISSCC Presentation