Praise be to Allah.
If the situation is as described, and this lady did not fast three days of Ramadaan out of heedlessness and not because she believed it was permissible to do so, then she has committed a serious major sin by violating the sanctity of Ramadaan. Fasting Ramadaan is one of the pillars of Islam, because Allaah says (interpretation of the meaning):
“O you who believe! Observing As-Sawm (the fasting) is prescribed for you as it was prescribed for those before you, that you may become Al-Muttaqoon (the pious).
[Observing Sawm (fasts)] for a fixed number of days, but if any of you is ill or on a journey, the same number (should be made up) from other days. And as for those who can fast with difficulty, (e.g. an old man), they have (a choice either to fast or) to feed a Miskeen (poor person) (for every day). But whoever does good of his own accord, it is better for him. And that you fast is better for you if only you know.
The month of Ramadan in which was revealed the Qur’aan, a guidance for mankind and clear proofs for the guidance and the criterion (between right and wrong). So whoever of you sights (the crescent on the first night of) the month (of Ramadan i.e. is present at his home), he must observe Sawm (fasts) that month”
So she must fast three days to make up for those days when she did not fast. If she had intercourse during the day on those three days when she did not fast, she must also offer expiation (kafaarah) for that day as well as making up the fast. If she had intercourse on two days then she must offer two expiations, as well as making up two days. The expiation is to free a slave; if that is not possible then she must fast for two consecutive months; if she is unable to do that then she must feed sixty poor persons from the local staple food. She must also seek the forgiveness of Allaah and repent to Him; she must observe the fasts that Allaah has enjoined on her and sincerely resolve not to break her fast in Ramadaan again. She must also feed one poor person for each of the three days because of her delay in making up the fasts until after the following Ramadaan.
And Allaah knows best.