Hello kdinino,

Welcome to Flyertalk.

There are maybe two ways you can get an upgrade. One--ask for it, although I don't know how likely it would be if you're Silver Elite, or second, accumulate at least 50 nights to reach Gold status. Once you reach this level, you can put in a request for a free upgrade whenever you make a reservation.

Since you have 3 nights already, adding the 15 night stay that you currently have would bring you up to 18 nights total. If your able to do so, you could earn an additional 15 nights annually just by applying for the Marriott Premier Visa Card. See the following thread for more info if needed: New Marriott Rewards Premier Visa Signature card.

If you apply for the Premier Card, you would earn 15 nights annually upon approval of the card (plus other benefits that will make having the card worth it). That would boost your "nights stayed" to a total of 33 nights. Then you would be just 17 nights away from reaching Gold Status.

Hope this helps.