Fireplaces should have filters and caps on them to keep sparks from traveling. They also keep rain out. Check the flashing around the chimney on your roof for leaks.
Three has to be a crack or a leak in a seal somewhere that the only way water can get into a structure that is built like that and it will be expensive to fix.
The water could be coming from a pipe that runs near the fireplace, in which case you should check the plumbing in your house. Or it comes from the chimney.