Whenever the Prophet ﷺ asked permission to enter, he knocked the door thrice with greeting and whenever he spoke a sentence (said a thing) he used to repeat it thrice. (See Hadith No. 261, Vol. 8).
عَنِ النَّبِيِّ ﷺ أَنَّهُ كَانَ إِذَا سَلَّمَ سَلَّمَ ثَلاَثًا وَإِذَا تَكَلَّمَ بِكَلِمَةٍ أَعَادَهَا ثَلاَثًا