Quikrete 90 lb- what am I doing wrong?

I am trying to form a 3" slab that is 2ftx2ft square in my backyard. I purchased 3 bags of Quikrete 90 lb. After mixing, I poured it into the form I made with 2x4s but the concrete mix is full of stones. So how do you get a smooth finish when its mostly stones and pebbles sticking up above the surface? What am I missing here???