the above said.... we know the arc is 3 units, meters or feet or who knows, and is in the Unit Circle, the Unit Circle is called like so because its radius is exactly 1 unit long.
[tex]\bf \textit{arc's length}\\\\
s=r\theta \quad
\begin{cases}
r=radius\\
\theta =angle~in\\
\qquad radians\\
------\\
r=1\\
s=3
\end{cases}\implies 3=1\theta \implies \cfrac{3}{1}=\theta \implies \stackrel{radians}{3}=\theta [/tex]
