Positive Definate Quadratic

Constrained Quadratic function

Quad[{x1_,x2_,x3_}]:=3 (((x1-x2))^2)+ 2 (((x1+3 x2+x3))^2)+ 2 x1 - x2 + x3 +4 (((x1- x2+2x3))^2)
c[1][{x1_,x2_,x3_}]:=1-((x1^2)+ 0.8(((x2+x3-0.5))^2)+0.9(((x3-0.2x2-0.5))^2))
c[2][{x1_,x2_,x3_}]:=0.9-x1- 0.5 x2 -Cos[x3]
c[3][{x1_,x2_,x3_}]:= -0.2+x1+x2 +Cos[0.5 x3 x2]
c[4][{x1_,x2_,x3_}]:= 1+x1-x2 -2 x3
RegionPlot3D[
And[
(* c[1][{x1,x2,x3}]>0, 
c[2][{x1,x2,x3}]>0,
c[3][{x1,x2,x3}]>0, *)
c[4][{x1,x2,x3}]>0
],
{x1, -2,2},
{x2,-2,2},
{x3,-2,2},
ColorFunction:>Function[{x1,x2,x3,f},Hue[((Quad[{x1,x2,x3}])/50)]],
PlotPoints->40
]

Important Question

cp={x1,x2,x3}/.NSolve[
D[Quad[{x1,x2,x3}],{{x1,x2,x3},1}]=={0,0,0}
][[1]]

{c[1][cp],c[2][cp],c[3][cp],c[4][cp]}