You could diplex the OTA signal into the Satellite signal but with 3 targets, you must diplex between the switch and the receiver and this introduces complexities. You will have to run the coax from the antenna to a splitter and then diplex each of the splits into the three sat cables.
It will probably be easier and more successful to run fresh RG-6 to each TV. Depending on the configuration and quality of the OTA splitter, you will loose up to 7.5dB of signal to at least 2 of the sites, thats 1/4 of the original signal, so a pre-amp might be in order unless your OTA towers are nearby. I have a similar situation and I used a Winegard HDP-269 pre-amp which works even with one station 2.5 miles from my antenna.