Error: No instance of constructor matches the argument list

Your problem with the first version is that your compiler is trying to prevent a bug.

Your problem with the second version is that you outsmarted your compiler and successfully managed to create a bug.

Given your constructor, you want to store pointers to the float values that are passed by reference. Since your second version now calls the constructor with references to the local variables float a, b, c;, you created an instance of Vector3DHeap which references them. But as soon as getUnitVector returns, those variables no longer exist and the references stored in Vector3DHeap became dangling references.

The solution is not to store pointers inside Vector3DHeap or to create copies of the parameters:

Vector3DHeap::Vector3DHeap(float x, float y, float z)
{
    this->x = new float(x);
    this->y = new float(y);
    this->z = new float(z);
}

Make sure that you properly delete the stored floats, though.