Kisame and Itachi are not at full strength. They are using Kage Bunshin, so their chakra is already cut in half, plus they are using chakra to channel the extraction jutsu. It is extremely feasible for them to lose 4 vs 1 in their current state.

And I didn't mean that they would keep Konoha occupied for the duration of the jutsu. Akatsuki obviously thinks they can win, so their intention is not to merely delay. But they will succeed in delaying konoha for at least a short while for the sake of drama. If they stopped them for the full 3 days then that would mean Akatsuki succeeds in doing whatever it is they intend to do to Gaara, and its likely that Konoha will succeed in somehow rescueing him. (whether or not he will still have the Shukaku remains to be seen).