There is no solution to that. You need to tell it to read 12 bytes, in any way that works out for you. For example,
fread(fid, [1 12], '*uint8')
fread(fid, [1 6], '*uint16', 'b');
fread(fid, [1 3], '*uint32', 'b');
There is no 'uint96' or equivalent. MATLAB does not have any data type which is 96 bits long.