GPIO 3 means "The third pin with General Purpose I/O functionality" - a generic pin that can be used for custom protocols. In the case of ESP8266, the pin is also used for I2S SD (data pin of the I2S bus), and this is what is actually used, because the I2S implementation in ESP8266 can work at 80MHz.

I don't seem to find the specifications of the I2S in Raspberry Pis, all I can find is in the range of kHz. So probably it cannot be done.

Not sure about frequencies nor NTSC but I've transmitted from Ohio to New Zealand with no amplifier (once) using WSPRRYPI and there is also https://github.com/F5OEO/rpitx