Can I protect 3 branch circuits with a GFCI outlet?
This is easy -- pigtail all the black (hot) wires together except for the one that's actually hot, and do the same with the white (neutral) wires.  Then, connect the hot black/white to the corresponding LINE terminals on the GFCI, and the pigtail black/white to the matching LOAD terminals on the GFCI.  Finally, pigtail all the grounds together and connect them to the GFCI's ground screw.

(If you don't want the rest of the circuit GFCI protected, leave a comment or edit your question.)