If there are not 2 cablecard slots in the back then it's not a Series 3 Tivo. Maybe what you actually have is a Tivo HD Series 3, which have them in the front behind a panel. The Series 3 has the OLED display in the front with a clock, while the Tivo HD is mostly blank.
Tivo made the Series 3 Tivo first, then later made another generation 3 Tivo they called the Tivo HD Series 3. They did this to bring the costs down. The Series 3 Tivo must use two cablecards if you want 2 tuners, doesn't matter if the cards are multi stream or not. The Tivo HD can get away with one multi stream card, to give you two tuners.
Tivo made the Series 3 Tivo first, then later made another generation 3 Tivo they called the Tivo HD Series 3. They did this to bring the costs down. The Series 3 Tivo must use two cablecards if you want 2 tuners, doesn't matter if the cards are multi stream or not. The Tivo HD can get away with one multi stream card, to give you two tuners.