You may be able to power a Pi3 from a USB port, but it is not recommended. The USB 2 standard specifies an output current of 0.5 Amp (USB3 can supply up to 1 Amp), well below the recommend rating for the Pi. This can lead to a variety of hard to track down issues (e.g. freezing of the Pi, SD card corruption, networking issues), a quick search of this site will turn up several more issues resulting from an insufficient power supply.
You could use a battery pack to power the Pi, but that does not really solve the problem as the battery will need to be recharged somehow, and is likely to lead to a subpar user experience when the battery dies mid movie or game.
One alternative may be to purchase a power bar (that has high Amp USB charging ports built in - these typically supply > 2.0 Amps) like this one
(Note: these come in a variety of socket and charging port configurations. This specific model only has 2 USB Ports that supply 2 Amps). This would only use one wall socket and supply sufficient current to safely power your Pi.