Factor $54=3^{2}\times 6$. Rewrite the square root of the product $\sqrt{3^{2}\times 6}$ as the product of square roots $\sqrt{3^{2}}\sqrt{6}$. Take the square root of $3^{2}$.
$$3\times 3\sqrt{6}-3\sqrt{72}-\sqrt{98}$$
Multiply $3$ and $3$ to get $9$.
$$9\sqrt{6}-3\sqrt{72}-\sqrt{98}$$
Factor $72=6^{2}\times 2$. Rewrite the square root of the product $\sqrt{6^{2}\times 2}$ as the product of square roots $\sqrt{6^{2}}\sqrt{2}$. Take the square root of $6^{2}$.
$$9\sqrt{6}-3\times 6\sqrt{2}-\sqrt{98}$$
Multiply $-3$ and $6$ to get $-18$.
$$9\sqrt{6}-18\sqrt{2}-\sqrt{98}$$
Factor $98=7^{2}\times 2$. Rewrite the square root of the product $\sqrt{7^{2}\times 2}$ as the product of square roots $\sqrt{7^{2}}\sqrt{2}$. Take the square root of $7^{2}$.
$$9\sqrt{6}-18\sqrt{2}-7\sqrt{2}$$
Combine $-18\sqrt{2}$ and $-7\sqrt{2}$ to get $-25\sqrt{2}$.
