The Prophet ﷺ said, "Night prayer is offered as two rakʿat followed by two rakʿat and so on, and if you want to finish it, pray only one rakʿa which will be witr for all the previous rakʿat." Al-Qasim said, "Since we attained the age of puberty we have seen some people offering a three-rakʿat prayer as witr and all that is permissible. I hope there will be no harm in it."
قَالَ النَّبِيُّ ﷺ صَلاَةُ اللَّيْلِ مَثْنَى مَثْنَى فَإِذَا أَرَدْتَ أَنْ تَنْصَرِفَ فَارْكَعْ رَكْعَةً تُوتِرُ لَكَ مَا صَلَّيْتَ قَالَ الْقَاسِمُ وَرَأَيْنَا أُنَاسًا مُنْذُ أَدْرَكْنَا يُوتِرُونَ بِثَلاَثٍ وَإِنَّ كُلاًّ لَوَاسِعٌ أَرْجُو أَنْ لاَ يَكُونَ بِشَىْءٍ مِنْهُ بَأْسٌ