It is, pricefan. Since Day One, I believe, it's been CBS and NBC and no one else. As in Day One in the current era (Modern). FOX added sports to go with the Simpson, Married with Children and whatever else was on their network in their 9th season, and immediately obtained the NFC package from CBS, four years before the latter would take the AFC package from NBC. NBC followed their 8-year NFL hiatus by taking over primetime football from ESPN. It's hard to believe that FOX has had football for more than a quarter century, but there you go!
The closest we've come to where all three aired simultaneously would have been every Week 17 since 2006 (except 2017) where both CBS & FOX would have the double-header. Both games usually last into FNIA (Football Night in America). Barring overtime, late national games end by 7:30 or 7:45, just in time for FNIA to get that final highlight in before their 8:20 kickoff.