Whenever the Prophet ﷺ spoke a sentence (said a thing), he used to repeat it thrice so that the people could understand it properly from him and whenever he asked permission to enter, (he knocked the door) thrice with greeting.
عَنِ النَّبِيِّ ﷺ أَنَّهُ كَانَ إِذَا تَكَلَّمَ بِكَلِمَةٍ أَعَادَهَا ثَلاَثًا حَتَّى تُفْهَمَ عَنْهُ وَإِذَا أَتَى عَلَى قَوْمٍ فَسَلَّمَ عَلَيْهِمْ سَلَّمَ عَلَيْهِمْ ثَلاَثًا