(Remember, the order the elements are written down in does not matter.) Also,

\begin{equation*} \{1, 2, 3\} = \{1, 1+1, 1+1+1\} = \{I, II, III\} = \{1, 2, 3, 1+2\} \end{equation*}

since these are all ways to write the set containing the first three positive integers (how we write them doesn't matter, just what they are).