A3000D mediator is compatible with A3000 motherboards that came out of desktop cases is the correct description which everyone failed to make clear, unofrtunately...especially Elbox.
The A3000D mediator only fits inside Mirage 3000 tower.
A4000Di mediator fits inside A4000D case (C= case) however A3000D equivalent of that product doesn't exist.
You will need a Voodoo 3 PCI card (2000 or 3000) in order to get RTG graphics through mediator, or at least S3 Virge PCI which is about on par performance wise with CV64/3D as they use exactly the same chipset. You can't just use any PCI gfx card with mediator as each one requires a specific driver written for it, and so far there's only drivers for Voodoo 3, 4, and 5 PCI, and S3 Virge PCI.
Your cheapest option is a Z2 ethernet card otherwise (prices taken from Elbox store):
Mirage 3000 Tower - 179.95 EUR
Mediator PCI 3000D - 289.95EUR
That's about 590USD before shipping.
And you would look for Ethernet, Sound, and Voodoo 3 elsewhere for better deals.