In a triangle the side a = 6 cm lies opposite the angle A = 30°. The radius of its circumcircle is:

The extended sine rule says a/sinA = $2R$. Here 6/sin30° = 6/(1/2) = 12, so $2R=12$ and $R=6$ cm. Stopping at 12 reports the diameter, and halving twice gives 3. Note that one side with its opposite angle is enough — the other two sides are not needed.