Dryer Outlet - 4 prong to 3 prong
Switch the cord, not the socket

You have a proper NEMA 14-30 dryer receptacle with a ground wire, which is good.  However, your "new" (old) dryer has a 3 prong (NEMA 10-30) cord.  This can be rectified -- get a 4 prong (NEMA 14-30) dryer cord and install it on the dryer in place of the existing cord, making sure to remove the neutral-ground bonding jumper in the dryer.