Shaykh of shias as-Saduq in his “Ilal ush-sharae” (p 318) narrated:
الصادق عليه السلام قال: قلت له لم صارت المغرب ثلاث ركعات وأربعا بعدها ليس فيها تقصير في حضر ولا سفر؟ فقال: ان الله عزوجل أنزل على نبيه صلى الله عليه وآله لكل صلاة ركعتين في الحضر فاضاف إليها رسول الله صلى الله عليه وآله لكل صلاة ركعتين في الحضر وقصر فيها في السفر إلا المغرب والغداة فلما صلى المغرب بلغه مولد فاطمة عليها السلام فاضاف إليها ركعة شكرا لله عزوجل، فلما أن ولد الحسن عليه السلام أضاف إليها ركعتين شكرا لله عزوجل فلما أن ولد الحسين (ع) أضاف إليها ركعتين شكرا لله عزوجل فقال للذكر مثل حظ الانثيين فتركها على حالها في الحضر والسفر.
Saduq (alaihi salam), it was said to him: Why Maghrib became 3 Rakahs, and 4 after it, there is no shortening in it, neither during staying in ones place, nor during the journey? He said: Allah Azwajal prescribed to his prophet Muhammad, sallalahu alaihi wa ali, 2 rakahs for each prayer during the stating at ones place. And the messenger (sallalahu alaihi wa ali) added to them 2 rakahs for each prayer, and shortening in them during the journey except in Maghrib, and al-Fajr. And when he prayed Maghrib, it was said to him that Fatima (alaihi salam) was born, and he added to it an rakah as an example of gratitude to Allah Azwajal. (and then it says that he added 2 rakah of shukr when was born Hasan and 2 when was borh Husayn).
So as it obvious, in accordance to this shia report, 3-d rakah to Maghrib, was added by messenger (sallalahu alaihi wa ala alihi wa sallam), when he was informed that Fatima was born. The problem for shias is, FACT, that Fatima (r.a) was born before 5 times prayer was prescribed for Muslims and most likely before the day of prophet hood was send.
Jafar Subhani said in his Message:
However we are of the view that the me’raj in which daily prayers were made compulsory took place after the death of Abu Talib which occurred in the tenth year of the prophethood of the Prophet.