We are required to mention the three symbols of Pesach.The Roasted Shankbone is a reminder for us that we placed lambs blood on the doors of our homes so that G-d would pass over "posach" our homes  when we were slaves in Egypt.  He then killed the first born sons of the Egyptians as the tenth plague.Matzoh is the flat, unleavened bread which we ate when ran from Egypt in a hurry.  There was no time to wait for the dough...