I think you're maybe overstating the degree of homogeneity in Java/C#/Python. E.g., CPython3 cannot run Python2 code at all, and there are a lot of platform-specific modules and behaviors in the stdlib ("import os"). I don't think Python has any real single standard to the level of detail that C does, although I may be mistaken.
For C, on approximately the same platforms, to approximately the same degree: either GCC or Clang would be the corresponding standard-compliant implementation.
CPython is looked to as the canonical Python. IronPython and PyPy are all modeled after it and aim to behave as closely to it as possible, while CPython is considered the gold standard. Comparing CPython3 and CPython2 is orthogonal to that; one is not trying to implement or emulate the other. You have similar situations with Mono C# imitating Microsoft C# and IronRuby imitating Ruby. If there is a difference, it is considered a deviation from Ruby which is the reference implementation.
In Java/C#/Python, you have a standard, a dominant implementation which is compliant to that standard. Few more independent/niche implementations which may not be 100% compliant. Do we have a similar implementation in C?