People have been successfully shooting weddings for many years with cameras much less capable than a D3000. Up until fairly recently, TLR cameras were very popular with wedding photographers but - and this is a big but - the good photos you see were likely taken by a good photographer who knew how to use his equipment properly. That's what you have to ask yourself.

You probably should sell the 70-300 out of your kit and buy a prime 35~50mm lens with a large maximum aperture to control depth of field in the portraits. I doubt the 18-55 kit lens has a large enough aperture.