Here a receptacle is added to a 3 way light circuit before the first switch. It is not controlled with the switches but is instead always hot. The source hot, neutral and ground are spliced to a 2-wire cable that runs to the new outlet. The 3 way switches and light are then wired in the usual way with the common on SW2 spliced to the source hot and the light hot wired to the common on SW1.
This drawing shows the wiring for adding a new light fixture using an always-hot receptacle outlet as the source. New cable is run from the receptacle to the new fixture location and a switch loop cable is run from there to the new switch location.
Here a 2-way push-button switch is wired to a lamp with 2 bulbs. This diagram can be used to rewire an old push-button lamp with a new switch replacement. The hot wire from the cord is connected directly to the black wire on the switch and the neutral is spliced to the neutral contact on each bulb sockets. The red and blue wires from the switch are each connected to the hot contact on one of the bulb sockets.
Here a new receptacle is added to a light fixture circuit. The connections at the light fixture are spliced and extended to the new receptacle. With this arrangement the new receptacle will turn off and on with the light fixture.