Convert HDMI video to Coaxial Antenna?
Assuming you want digital channel 3, you can use an HDMI to QAM modulator, like this one or this one. You will need appropriate multiplexers, amplifiers, and splitters to combine your feed with your CATV provider and then distribute to the TVs.

If you need analog channel 3, then it is more complicated. You will probably first need to convert to SDI, then convert to analog video and audio, and finally feed an analog modulator.