Max 5D3 ISO is 102400
Rumors (and announcements) so far have used the maximum analog ISO, not the digital "High mode" value (because that's not actually an ISO; it's still the maximum analog ISO but underexposed and pulled up in post).
Having a 51200 maximum makes me think that they will use the sensor design from 1DX. If the official price will be 2000 USD, I'm interested again.
I think "maximum" is defined as expanded and not native ISO, as in "H1" and "H2". There is not way that a cheaper FF body will have a better native ISO range than the 5D3. Instead, it will probably be ISO 100-12800 native w/ H1-25600 and H2 - 51200.