Hello Ernest,
Great question, It sounds like you might need a couple of solutions. For the Announcements it may suffice by putting the video / audio file for announcements on a USB drive that you can play using the TV’s media player, you should be able to loop that if it is something you would like to have playing on repeat.
For the presentations you could use something like this which is a HDMI to RF modulator, which would allow you to use any pre existing coax cable that you may already have run in the building.
Alternatively if the distance between your PC and the TV’s are within 200 Ft you could use one of these HDMI wireless devices that can send to signal to 3 destinations.