[不等式] weird one

For $x,y,z>0, c>0$ with $ x+y+z=3 $ find the Min Value of P
\[ P=\frac{1}{\sqrt{x^2+y+c}}+\frac{1}{\sqrt{y^2+z+c}}+\frac{1}{\sqrt{z^2+x+c}} \]