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If you have a colour image, such as

img = ImageResize[ExampleData[{"TestImage", "Apples"}], 100]

As mentioned by JM, there is ImageHistogram

ImageHistogram[img]

But for some reason you have NOT explained, you want to use Histogram3D, which takes arguments in the form of list of pairs of values.

Then the number of colour channels is 3

ImageChannels[img]

That implies that the dimensions of the data array, is the dimensions of the pixel array and depth 3.

Dimensions[ImageData[img]]
(* {67, 100, 3} *)

So the data is incompatible.

Of course you could rearrange the data.

Partition[Flatten[ImageData[img]], 2]

or

ArrayReshape[
 ImageData[img]
 , {Times @@ Dimensions[ImageData[img]]/2, 2}
 ]

but then you are mixing the RGB values and

{{r[1], g[1], b[1]}, {r[2], g[2], b[2]}, {r[3], g[3], b[3]}, {r[4], 
  g[4], b[4]}, {r[5], g[5], b[5]}, ... , {r[n], g[n], b[n]}}

becomes

{{r[1], g[1]}, {b[1], r[2]}, {g[2], b[2]}, {r[3], g[3]}, {b[3], 
  r[4]}, ... , {g[n], b[n]}}