Try the followings: (with editing below)
Normalization just divide by 255 for uint8 image.
[Ny,Nx,Nc] = size(input_image_3D); % Ypur input_image_3D, size is 64x64x3
J = zeros(size(input_image_3D)); % Initialize matrix J
for k =1:Nc
I = double(input_image_3D(:,:,k));
J(:,:,k) = I/255; % Perform normalization channel by channel
end
J = reshape(J,Ny,[],1); % Reshape to 2 dimensions first
J = reshape(J,[],1); % Reshape to a column vector
%% No commnent below, same as your code
%
%
fid=fopen('filename.txt','wt');
fprintf(fid,'%0.16f\n',J);
fclose(fid);
% The txt file is a column vector and hecnce you need to revert back to
% its original size. Also use imshow instead of imshow3D
%
testingReadFile=importdata('filename.txt'); % Import the txt file
revertI = reshape(testingReadFile,Ny,[]); % Reshape back to 64 rows first
revertI = reshape(revertI,Ny,[],Nc); % Revert back to 3 channels
imshow(revertI,[]) % Showing the image
