= Solution
The three <elementary row operations> are:
* interchange two distinct rows, $R_i\leftrightarrow R_j$;
* multiply a row by a nonzero <scalar>, $R_i\leftarrow cR_i$ with $c\ne0$;
* add a <scalar multiple> of another row, $R_i\leftarrow R_i+cR_j$ with $i\ne j$.
The nonzero requirement in the second operation ensures that each <elementary row operation> is reversible.
$$
\boxed{R_i\leftrightarrow R_j;\qquad R_i\leftarrow cR_i\ (c\ne0);\qquad R_i\leftarrow R_i+cR_j\ (i\ne j).}
$$
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