I want to move only the Z axis at the midpoint of the 3D parabola connecting the two points.

Vector3_controlPoint=new Vector3(0,0,-50);
var control=(_trans1.position+_trans2.position)/2+_controlPoint;
verticalPoints=_middlePoints+2;
List<Vector3>l_list=new List<Vector3>();
l_list.Add(_trans1.position);
for (inti=1;i<=_middlePoints;i++)
{
    start=(float)i/(float)(totalPoints-1);
    Vector3_pos1 = Vector3.Lerp(_trans1.position, control, t);
    // Interpolate along line S1:S1 = end-control;
    Vector3_pos2 = Vector3.Lerp(control,_trans2.position,t);
    // Interpolate along line S2—Q1-Q0
    Vector3_pos = Vector3.Lerp(Q0,Q1,t);
    l_list.Add(mpos);
}
l_list.Add(_trans2.position);

var control=(_trans1.position+_trans2.position)*0.85f+_controlPoint;