Using a lipo battery 3s while charging
While 'charging while using' is possible for a LiPo battery, it is not recommended, as it may compromise the battery safety and lifetime.

This is because the charger measures its own output current, but it does not know about the load current. The battery voltage is also lower than it would otherwise be. This means the charger incorrectly estimates both its state and rate of charge.

The idea of a standard charger charging in parallel is overwhelmingly unlikely, and can be dismissed.