The 5D3 is easily 1 stop ISO better than the 1D3. The 1D3 and 7D are very close, but the 1D3 retains more detail. Any of the latest bodies do better than the 1D3 these days, from the 7D2, 6D, 1DX, 5D3 and the new high res 5DS/R bodies.
Honestly if I needed the best blend of the 5D2 and 1D3, I would look for a lightly used 1D4. IMO it retains the best features of both cameras at this point, with FPS, AF and ISO performance.
The 7D2 and 5D3 make a great pair, the 7D2 is quite useful at ISO 12800 as well though. So if you are thinking about just swapping out the 1D3 for a 7D2, that could be an option. The 7D2 is better than the 1D3 in almost all regards (battery life). It has auto-ISO in manual mode fully configurable, it has a better AF system, it focuses in low light better, and once you equalize image sizes between the two, the 7D2 is probably about 2 stops better (ditto with the 5D3).
Showing AF and ISO 12800: