There is a whole section on visual signals for downed aircraft in the AIM (Airman's Information Manual), ranging from hand/body signals to ground markers. But 3 fires is not mentioned. It does mention that if you spot a downed aircraft, to "transmit the information to the nearest FAA or other appropriate radio facility". There are always several frequencies you can call in on for assistance, I'd start with the universal emergency frequency, 121.5 mHz.

By the way, the "require assistance" ground signal is a "V", made with any material available, at least 10 feet or larger. An "X" means "require medical assistance", although I'd be careful with that one, since a "yellow cross" means the crash has already been reported and identified.