Neil,
Your first bird is first winter Heuglin's because of its dark primaries with no hint of paler window on inner primaries and its medium width tail band [too wide for Mongolian]. It has a very heavy bill but this is still within range for Heuglins.
The second bird is striking because of its paleness. The mantle is too pale for either Heuglin's or Vega but fits Mongolian, as do the small scattered dark markings on the mantle etc. Also the more rounded head shape fits Mongolian whereas Heuglin's or Vega would have flatter heads with more sloping foreheads.
The last bird is a bit trickier because it shows a clear paler panel on its inner primaries. However it is still closest to Heuglin's because of the darker outer greater coverts. Also the tail band does not appear to be narrow enough or clear-cut enough for Mongolian. It is not Vega because Vega's paler panel on the inner primaries is caused by the pale areas extending onto the outer web [ie both webs] of each feather whereas the paleness on your bird is only on the inner web. I therefore think it is taimyrensis but with stronger vegae features than many. Indeed the small pale lines appearing on the dark outer webs of the inner upper primaries may be hybrid features.
Mike Chalmers
