$ 3 \lt \sqrt{10} \lt 4 \Rightarrow 3-\sqrt{10} \lt 0 \Rightarrow | \frac{3-\sqrt{10}}{2\sqrt{10}} | = -\frac{3-\sqrt{10}}{2\sqrt{10}} \\ \frac{-4}{\sqrt{10}} \lt 0 \Rightarrow | \frac{-4}{\sqrt{10}} | = -(\frac{-4}{\sqrt{10}}) = \frac{4}{\sqrt{10}} \\ 3 \lt \sqrt{10} \lt 4 \Rightarrow 4-\sqrt{10} \gt0 \Rightarrow | \frac{4-\sqrt{10}}{3} | = \frac{4-\sqrt{10}}{3} \\  D= | \frac{3-\sqrt{10}}{2\sqrt{10}} | + | \frac{-4}{\sqrt{10}} | + | \frac{4-\sqrt{10}}{3} |  = \frac{-9+3\sqrt{10}+24-8\sqrt{10}+20}{6\sqrt{10}} \\ = \frac{35-5\sqrt{10}}{6\sqrt{10}} = \frac{35-5\sqrt{10}}{6\sqrt{10}} \times \frac{\sqrt{10}}{\sqrt{10}} = \frac{35\sqrt{10}-50}{60} = \frac{7\sqrt{10}-10}{12} $