Coding Practice - Float* -> Vector3* cast
I would go with a 3rd option that is closest to option 2. But instead of taking the three parameters separately in the constructor, take an array of floats.

struct vec {
    float x,y,z;
    vec(float fv[]) : x(fv[0]), y(fv[1]), z(fv[2]) {}
};

int main() {
    float verts[] = {1,2,3,4,5,6,7,8,9};

    for (int i = 0; i &lt; 9; i += 3) {
        const vec a(&amp;verts[i]);
        // ...
    }

    return 0;
}