How can I have an axis label automatically positioned below the X-axis?

You can use Frame->True

Histogram[{1, 2, 3}, Frame -> True, FrameLabel -> {{"Y", None}, {"X", "my histogram"}}, 
  RotateLabel -> False]

Mathematica graphics

I wish there was a way of doing it without the frame - Karolis

Labeled[Histogram[{1, 2, 3}], {"X", "Y"}, {Bottom, Left}]

Mathematica graphics

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