3. Regardless of the value of $m$, the vertex of the parabola
$$
y=x^{2}+2 m x+m^{2}+m-1
$$

always lies on a straight line. Then the function expression of this straight line is $\qquad$ .
3. $y=-x-1$.

Transform the quadratic function into $y=(x+m)^{2}+m-1$, knowing that the coordinates of the vertex of the parabola are $\left\{\begin{array}{l}x=-m, \\ y=m-1 .\end{array}\right.$ Eliminating $m$, we get $x+y=-1$.