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.
Writing words of divorce on paper is considered a metaphor for divorce, so divorce does not take place unless one intends it; this is our view in Islamweb. So, if the husband wrote these words "you are divorced thrice" on a paper while intending to really divorce his wife, then divorce has taken place thrice, according to the view of the majority of scholars. Ibn Taymiyyah, on the other hand, is of the view that pronouncing the words of divorce three times in one sitting is considered one divorce.
Therefore, based on the view of the majority of scholars, irrevocable divorce has taken place and his wife has become forbidden to him unless she marries another man. Allah The Almighty Says (what means): {And if he has divorced her [for the third time], then she is not lawful to him afterward until [after] she marries a husband other than him. And if the latter husband divorces her [or dies], there is no blame upon the woman and her former husband for returning to each other if they think that they can keep [within] the limits of Allah. These are the limits of Allah, which He makes clear to a people who know.} [Quran 2:230] However, according to Ibn Taymiyyah, only one revocable divorce has taken place, unless it was preceded by two other divorces.
Assuming that the divorce has taken place, then the waiting period ('Iddah) starts from the moment the husband wrote the words of divorce on paper while intending it. The fact that his wife didn't know it until a month later doesn't affect the ruling.
Allah Knows best.