Area-51m R2, upgrading storage to three M.2 SSD

Folks, I have an Area-51m R2 which came with 1TB M.2 SSD. I want to add two 2TB Samsung 980Pro M.2. Documentation says up to three M.2 SSDs. But when I open up the case I can only see one additional slot (SSD-B). The documentation is fairly bad, no photos, no breakdown of options etc. so I can't even see what I'm missing here! And the Premium support I pay for told me to go away when I asked them what my options were for upgrading storage. Any help appreciated!