If angle ratio is $1:2:3$, the angles are:
\[
x,\ 2x,\ 3x \Rightarrow x + 2x + 3x = 180^\circ \Rightarrow x = 30^\circ
\Rightarrow \text{Angles: } 30^\circ,\ 60^\circ,\ 90^\circ
\]
The side ratios in a triangle with angles $30^\circ$, $60^\circ$, $90^\circ$ are:
\[
1:\sqrt{3}:2
\]