Ta có: \(x^2+xy+y^2=\frac{1}{2}\left(x+y\right)^2+\frac{1}{2}\left(x^2+y^2\right)\ge\frac{1}{2}\left(x+y\right)^2+\frac{1}{4}\left(x+y\right)^2=\frac{3}{4}\left(x+y\right)^2\)
\(\Rightarrow M\ge\sqrt{\frac{3}{4}\left(a+b\right)^2}+\sqrt{\frac{3}{4}\left(b+c\right)^2}+\sqrt{\frac{3}{4}\left(c+a\right)^2}\)
\(\Rightarrow M\ge\sqrt{3}\left(a+b+c\right)=3\sqrt{3}\)
\(M_{min}=3\sqrt{3}\) khi \(a=b=c=1\)