Charging Li-Ion Battery in Series
The BQ24105 can handle multicell battery charging as stated in the datasheet. The IC just takes care of the charging of the batteries but assumes that the battery pack (the dashed box in Figure. 1) has all the necessary protection circuit.

A battery pack with multiple batteries in series should have a protection circuit that monitors battery voltage, temperature and current. A balancing circuit is also necessary to monitor each cell in the pack to equalize the voltage for all cells.

Even if you buy the same type of batteries from a single manufacturer, each cell has small variations causing them to charge and discharge differently which makes it necessary to have a balancing circuit.