To find side $ a $ use formula:
$$ r = \dfrac{ \sqrt{ 3 } \cdot a }{ 2 } $$After substituting $ r = 3 $ we have:
$$ 3 = \dfrac{ \sqrt{ 3 } \cdot a }{ 2 } $$ $$ \sqrt{ 3 } \cdot a = 3 \cdot 2 $$ $$ \sqrt{ 3 } \cdot a = 6 $$ $$ a = \dfrac{ 6 }{ \sqrt{ 3 } } $$ $$ a = 2 \sqrt{ 3 } $$