No fasting counts for the one who fasts continuously. Fasting three days is like fasting the entire year

The Prophet (may Allah's peace and blessings be upon him) was informed that ‘Abdullah ibn ‘Amr (may Allah be pleased with him and his father) was fasting continuously without breaking his fast throughout the year and was praying the entire night without sleeping. So, he forbade him from doing that and said to him: Fast and break your fast, and stand in prayer and sleep.

He forbade him from fasting continuously and standing in prayer all night, and said to him: If you do that, your eyes will become weak and sunken, and your soul will become exhausted and weary. The one who fasts the entire year has not truly fasted, as he does not obtain the reward of fasting due to violating the prohibition, and he does not experience the benefit of breaking the fast, as he is constantly abstaining.

Then he guided him to fasting three days of each month, as it is equivalent to fasting the entire year, because each day is counted as ten days, which is the minimum multiplication of a good deed. ‘Abdullah said: Indeed, I am capable of more than that.

He (may Allah's peace and blessings be upon him) said: Then fast the fast of David (peace be upon him), which is the best type of fast, as he used to fast every other day and he would not flee when facing the enemy because the way he adopted in fasting did not weaken his body.