The best replacement for d3x is d3s.

The best alternatives to d3x are rated as following:
★★★★★  d3s
★★★★  d700
★★★  d4
★★★  d800
★  1dx
★  1ds mark iii
★  canon 1ds mark iii
★  mark iii
★  a900
★  1ds
★  nikon d3s
★  d3
★  nikon d800
  d5x
  phase one
  nikon d4
  5d mark ii
  d4s
  sony a900
  1d mark iv
  d800e
  mkiii

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Speaking about d3x, one probably shouldn't forget about d4.

Other search terms related to d3x and their ratings are:
★★★★★ d4
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