I say, and with God's help: If you do not sit at the end of the third rak'ah, your prayer is invalid as a mandatory prayer and becomes a voluntary one. Therefore, you must repeat the obligatory Maghrib prayer. If you sit at the end of the third rak'ah, you are required to perform the prostration of forgetfulness, and it is preferable to perform a fifth rak'ah so that the last two rak'ahs are voluntary. And God knows best.