private static long load3(byte[] in, int idx) {
    long result;
    result = (long) in[idx] & 0xff;
    result |= (long) (in[idx + 1] & 0xff) << 8;
    result |= (long) (in[idx + 2] & 0xff) << 16;
    return result;
  }
Returns 3 bytes of {@code in} starting from {@code idx} in Little-Endian format.
java_src/src/main/java/com/google/crypto/tink/internal/Ed25519.java