I want to add a receptacle to a box I have in my pantry that has an existing light switch. I have a three wire load coming in to the box from the circuit breaker. The red is wired to the switch and the blacks and whites are connected with wire nuts. The three wires feed other boxes in my kitchen. My problem is how to connect the third white wire to the receptacle with only two screws for the white wires. What's the proper way to wire the three white wires?

One way is to add a short fourth white wire. Use a large enough wire nut to connect all 4 wires, and run the short wire to the receptacle.

If the red and black wire are different circuits with a shared neutral, be sure and shut off both circuits before working on the neutral.

If it is 14 wire and the receptacle is fairly new you should be able to just backstab another wire into the small holes in the back of the device. If it is 12 wire then follow Labman's instructions.

I should have mentioned that. It will work even if only one wire is #14. Use the screws for the other 2. Back stab mean pushing the wires into little holes on the back near the screws.