How can I wire a 3 way switch with power feeds to another switch?
To wire to other loads from the box with a 3-way switch and a line feed, you simply identify the line cable, that is, the black line hot (always hot) and its associated white neutral (both being in the same cable).

Then connect another cable to the line hot and neutral routing it to the location you want to power-- connecting black to black, white to white and gnd to gnd. If your current connections are with wire nuts you must unscrew the nut on the line connections, add the new wire and reconnect with the same wire nut or a larger one if necessary.