In the olden days each of the three hours was live/new and that's how Joey pitched it to Andy Lack.
Depending on what was going on in the news, they did recaps during the second and third hours but the third hour wasn't a mashup like it is now. Now, it takes a big damn news day to get three live hours out of these two.
Honestly? It seemed like the third hour changed when their affair was taking off. It was slow, it was incremental, it was business as usual during the first two hours but during the occasional third hour Mika changed her clothing or her hair was rearranged. She worked most of the third hour but he didn't and vice versa.* Then one week, it seemed like, bam - no more live third hour at all and after that there was no turning back. Now, it's rare enough that viewers notice it: "Hey, was the third hour live today? It seemed like it was."
*This was during the time frame when it was leaked that they were sharing a dressing room. Before the divorces, before they went public.