how can I convince Border Police to let me stay in Poland for 3 months ?
You can't. One of the conditions is that you will leave the Schengen zone after three months, and it is painfully obvious that you have no intention to do so. No consul will ever believe otherwise, even if you do find someone foolish enough to sponsor you.
Sorry, but you stink of high risk and dishonesty, and there's nothing you can do to wash the stink out. No amount of money, and no amount of sponsors will help. Scratch the EU off of the list of places you will visit in your lifetime.