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  

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  

"When the Messenger of Allah ﷺ would give the Salam he would do so three times, and when he would say a statement, he would say it three times."  

أَنَّ رَسُولَ اللَّهِ ﷺ كَانَ إِذَا سَلَّمَ سَلَّمَ ثَلاَثًا وَإِذَا تَكَلَّمَ بِكَلِمَةٍ أَعَادَهَا ثَلاَثًا  

"When the Messenger of Allah ﷺ would give the Salam he would do so three times, and when he would say a statement, he would say it three times." (Using translation from Tirmidhī 2723)   

أَنَّ رَسُولَ اللهِ ﷺ كَانَ إِذَا تَكَلَّمَ بِكَلِمَةٍ رَدَّدَهَا ثَلَاثًا وَإِذَا أَتَى قَوْمًا فَسَلَّمَ عَلَيْهِمْ عَلَيْهِمْ سَلَّمَ ثَلَاثًا  

"When the Messenger of Allah ﷺ would give the Salam he would do so three times, and when he would say a statement, he would say it three times." (Using translation from Tirmidhī 2723)   

«أَنَّ رَسُولَ اللَّهِ ﷺ كَانَ إِذَا تَكَلَّمَ بِكَلِمَةٍ أَعَادَهَا ثَلَاثًا لِتُعْقَلَ عَنْهُ»