When you have an array of higher dimensions you will also have new axes. For example, in a dimension 3 array (e.g. a cube) you will have 3 axes (row, column, depth).
When you pass the axis in the np.argmax
you are telling numpy along which axis you want the maximum argument. 3 will throw an error because your array only has 3 axes (0, 1, 2).
Here is an article about numpy arrays axes.
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