\[\boxed{\text{560\ (560).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[a_{n} = 3 \cdot n \Longrightarrow 3;6;9;12;15;\ldots\]
\[a_{1} = 3,\ \ a_{5} = 3 \cdot 5 = 15,\ \ \]
\[a_{10} = 3 \cdot 10 = 30,\ \]
\[a_{100} = 3 \cdot 100 = 300,\ \ \]
\[a_{n} = 3 \cdot n.\]