3 of 3 added 163 characters in body

MATL, 5 bytes

3Z%Bz

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Exact transliteration of my MATLAB answer. Note that input and output are implicit. -2 bytes thanks to Luis Mendo.

3Z%   % Typecast: changes input (implicitly taken and converted to double) to uint64 without changing underlying bits
B     % Convert integer to array of 1s and 0s
z     % Count nonzero entries