Factor $90=3^{2}\times 10$. Rewrite the square root of the product $\sqrt{3^{2}\times 10}$ as the product of square roots $\sqrt{3^{2}}\sqrt{10}$. Take the square root of $3^{2}$.
$$3\sqrt{10}-\sqrt{40}-\sqrt{10}$$
Factor $40=2^{2}\times 10$. Rewrite the square root of the product $\sqrt{2^{2}\times 10}$ as the product of square roots $\sqrt{2^{2}}\sqrt{10}$. Take the square root of $2^{2}$.
$$3\sqrt{10}-2\sqrt{10}-\sqrt{10}$$
Combine $3\sqrt{10}$ and $-2\sqrt{10}$ to get $\sqrt{10}$.
