how to go underwater

In my swimming class, our coach asked us to go three feet underwater with streamline and dolphin kick, then go above water, then go three feet underwater, then above water..... I found it hard to go underwater. I could swim three strokes(not butterfly), however, I just cannot swim as the coach asked. I felt my body float back to the surface of the water. can anyone share some instructions/ articles/ advices? Thanks a lot.