X.C.
If the couch cushions come off, take them off and build tunnels, forts, etc with them. Make it firehouse themed if you want - say they have to crawl into the "burning building" to put out the fire.
Fireman Sam - great firefighter series for kids that age - my son loved him when he was three. We had it on-demand on the PBS Kids Sprout channel.
Make a pretend hose out of rope or paper towel tubes or whatnot. Pretend to put out fires all around. Designate a couch to be the firetruck. Designate a room to be the firehouse. Have them wait to hear an alarm (ie you) and then they have to pretend to put on boots, jacket, etc and get in firetruck and "drive" to fire. Then put fire out with pretend hose. Then drive back to firehouse and have them "make dinner" and go to sleep. Dont forget to have a place (door jam maybe) that htey can pretend to slide down a pole (ie just move from standing to squatting while pretending to hold on).
Cut out the shape of a firetruck from construction paper and have them use crayons, paint, stickers, etc to decorate it.