3. Given
$$
a=-\frac{2019 \times 2019-2019}{2018 \times 2018+2018}, \quad b=-\frac{2020 \times 2020-2020}{2019 \times 2019+2019}, \quad c=-\frac{2021 \times 2021-2021}{2020 \times 2020+2020} \text {, }
$$

then $a b c=$ . $\qquad$
answer: -1