Accessing slice of 3D numpy array

I have a 3D numpy array of floating point numbers. Am I indexing the array improperly? I'd like to access slice 124 (index 123) but am seeing this error:

>>> arr.shape
(31, 285, 286)
>>> arr[:][123][:]
Runtime error 
Traceback (most recent call last):
  File "<string>", line 1, in <module>
IndexError: index 123 is out of bounds for axis 0 with size 31

What would be the cause of this error?