\(\sqrt{12-3\sqrt{7}}=\frac{\sqrt{2}\sqrt{12-3\sqrt{7}}}{\sqrt{2}}\)
\(=\frac{\sqrt{24-6\sqrt{7}}}{\sqrt{2}}=\frac{\sqrt{24-2.3.\sqrt{7}}}{\sqrt{2}}\)
\(=\frac{\sqrt{24-2\sqrt{63}}}{\sqrt{2}}=\frac{\sqrt{21-2\sqrt{21}\sqrt{3}+3}}{\sqrt{2}}\)
\(=\frac{\sqrt{\left(\sqrt{21}-\sqrt{3}\right)^2}}{\sqrt{2}}=\frac{\sqrt{21}-\sqrt{3}}{\sqrt{2}}\)
\(=\frac{\sqrt{42}-\sqrt{6}}{2}\)