Replacing and outlet (2 plugs) with only one black, one-white, has ground wire
The first outlets you replaced had a black wire, white wire and ground  coming in, feeding the outlet and then leaving the outlet to go to another outlet, feeding it and going somewhere else. The last outlet on the circuit will only have a black wire, white wire and ground coming in to feed the outlet and will end there. Just hook the single black wire to the brass screw and the white wire to the silver screw and the ground wire to the green screw. 

When replacing any outlets or switches, do not utilize the backstabs as they are prone to failure. Use the screw terminals and shepherd hook the wire around them.... and turn off the power

If the outlets you're replacing utilize four screws on the outlets, you can pigtail the wires and only use two screws. This will make future replacements easier and will provide more reliable service.