The issue of Raka'ah in Witr is quite flexible. It doesn't have to be only three. It is reported from the Messenger (salallahu alayhi wassalam) that he performed Witr in various ways and that he commanded similarly. For example,
Duwaid bin Nafi' said: " Ibn Shihab informed me, saying: 'Ata bin
Yazid narrated to me from Abu Ayyub: That the Prophet (sallallahu
'alaihi wa sallam) said: 'Witr is a duty, and whoever wants to pray
witr with seven (rak'ahs), let him do so; whoever wants to pray witr
with five, let him do so, whoever wants to pray witr with three, let
him do so; and whoever wants to pray witr with one, let him do
so.'"(Sahih)
أَخْبَرَنَا عَمْرُو بْنُ عُثْمَانَ، قَالَ حَدَّثَنَا بَقِيَّةُ، قَالَ
حَدَّثَنِي ضُبَارَةُ بْنُ أَبِي السَّلِيلِ، قَالَ حَدَّثَنِي دُوَيْدُ
بْنُ نَافِعٍ، قَالَ أَخْبَرَنِي ابْنُ شِهَابٍ، قَالَ حَدَّثَنِي
عَطَاءُ بْنُ يَزِيدَ، عَنْ أَبِي أَيُّوبَ، أَنَّ النَّبِيَّ صلى الله
عليه وسلم قَالَ " الْوِتْرُ حَقٌّ فَمَنْ شَاءَ أَوْتَرَ بِسَبْعٍ
وَمَنْ شَاءَ أَوْتَرَ بِخَمْسٍ وَمَنْ شَاءَ أَوْتَرَ بِثَلاَثٍ وَمَنْ
شَاءَ أَوْتَرَ بِوَاحِدَةٍ " .
and
Ibn 'Umar reported Allah's Messenger (way peace be upon him) as
saying: Witr is a rak'ah at the end of the prayer. source
حَدَّثَنَا شَيْبَانُ بْنُ فَرُّوخَ، حَدَّثَنَا عَبْدُ الْوَارِثِ، عَنْ
أَبِي التَّيَّاحِ، قَالَ حَدَّثَنِي أَبُو مِجْلَزٍ، عَنِ ابْنِ عُمَرَ،
قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " الْوِتْرُ رَكْعَةٌ
مِنْ آخِرِ اللَّيْلِ " .
Witr is a odd numbered stressed Sunnah (obligatory according to Ahnaf[those who follow the Hanafi Madhab]) and you can perform it in any one of the ways as it is reported from the Prophet (salallahu alayhi wassalam).