It's about Ramadan, I have broken the fast for 3 days and I want to ask God to forgive me, knowing that the Prophet said that if you fast a whole year it won't match one day in Ramadan. What can I do then? How can I ask God for forgiveness?
All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
With regard to the ruling on breaking the fast during the day of Ramadan, then if one breaks it by having sex, he has to make up the missed day and expiate for it. The expiation is as mentioned in the Hadeeth. It was narrated on the authority of Abu Hurayrah that he said, “While we were once sitting with the Prophet, sallallaahu ‘alayhi wa sallm, a man came and said, ‘O Messenger of Allah, I am ruined.’ The Prophet asked: ‘What is it?’ He replied: ‘I had sexual intercourse with my wife while I was fasting.’ The Messenger asked him: ‘Can you afford to manumit a slave?’ He replied in the negative. The Prophet asked him: ‘Can you fast for two successive months?’ He also replied in the negative. The Prophet , asked him: ‘Can you afford to feed sixty poor persons?’" He replied in the negative. In the meantime a man from the Ansaar came with a basket full of dates. The Prophet said to the man, "Take it and give it in charity (as expiation for your sin)." The man said "Should I give it to some people who are poorer than me and my family, O Allah's Apostle? By Him Who has sent you with the truth, there is no family between Medeenah's two mountains poorer than me and my family.” [Al-Bukhari and Muslim]
However, if one breaks the fast by deliberately eating or drinking, then he has to make up that day and ask Allah's Forgiveness for violating the sacredness of the month of Ramadan. This is the opinion of the majority of the Muslim scholars . Finally, the Hadeeth which you referred to in the question is reported by Imam Abu Dawood and At-Tirmithi in their books of Sunnan.
For more benefit, please refer to Fatwa 82187.
Allah knows best.