The 75ohm input is for an OVA antenna.
Get an HDMI input selector if you need more HDMI inputs. That will get you an HD picture. Other options won't.
You could get an active HDMI to AV converter and connect that to a channel 3 RF modulator but the picture will look pretty bad and the TV has to have an NTSC analog tuner built in.
If you get a Roku Express+ that has AV outputs so you could connect it directly to the AV inputs of the TV or to the modulator.