public static class Vector3Extensions
{
    public static void SetX(ref this Vector3 v, float x) { v.Set(x, v.y, v.z); }

    public static void SetY(ref this Vector3 v, float y) { v.Set(v.x, y, v.z); }

    public static void SetZ(ref this Vector3 v, float z) { v.Set(v.x, v.y, z); }

    public static void AddX(ref this Vector3 v, float x) { v.Set(v.x + x, v.y, v.z); }

    public static void AddY(ref this Vector3 v, float y) { v.Set(v.x, v.y + y, v.z); }

    public static void AddZ(ref this Vector3 v, float z) { v.Set(v.x, v.y, v.z + z); }
}

위 코드를 아무 파일에나 넣는다

사용예)

Vector3 v = new Vector3(1f, 1f, 1f);

v.SetX(3f);
Debug.Log(v); //(3f, 1f, 1f)

v.AddX(-1f);
Debug.Log(v); //(2f, 1f, 1f)