Is that not what across the board means? There are 3 news channels, all of them have to be added together for an across the board measurement.
“Across the board” means something closer “true in every case”. Though as I type this, I realize some people my think of it as “in total.”
“across the three major cable news networks”
Which I paraphrased as
“Across all the channels”
Which the person replying to me changed to
“Across the board”.