It might be possible to get close since you get 5 points for the credit card and another 3 for being platinum. That would be another 8 points per dollar spent at a FS Marriott for the room rate. You would also get 5 points per dollar charged for room tax. Your calculation assumes 1000 paid nights at $125/night. At that rate you would get another 1 million points, excluding the amount received from charging the room tax to your card.

Adding in promotions and extra points from the room tax (assuming the tax is 10 % of room rate, you would get 125 * 100 *5 or another 62,500 points) could get you the 750,000 needed to reach 3 million. If you take points at FS properties for the platinum gift, you could pick up another 100,000 points for 200 stays at a FS property.