Pigtailing would be the easiest. Cut about a 6" length of the white
conductor from a piece of Romex. Connect the three wires in the outlet
box and one end of the pigtail with a wirenut - check the box to make
sure that the size wirenut you are using can handle four 14 gauge
conductors (assuming you are using 14 gauge/15 amp line). Follow the
instructions with the wirenuts for strip length too. Pull all wires
to make sure they are tight after twisting. Connect the other end of
the pigtail to one of the Neutral screws on the outlet. I would do the
hot (black) side the same way. It will be easier to stuff everything
back into the outlet box too since you can pack the wires all the way
to the back, then only have two wires connected to the outlet to guide
in.