Answer: R, D, S are not collinear.
Three points are collinear if the sum of two of the distances equals the third.
\[RD=\sqrt{2^2+(-2)^2}=\sqrt8=2\sqrt2\approx2.83\]
\[DS=\sqrt{1^2+(-2)^2}=\sqrt5\approx2.24\]
\[RS=\sqrt{3^2+(-4)^2}=5\]
\(RD+DS\approx5.07\ne5\), so the points are not collinear.