Shaykh Muhammad al-Saalih al-‘Uthaymeen said:
It is permissible to pray Witr with three rak’ahs, or with five, or with seven, or with nine. If a person prays Witr with three, it may be done in one of two ways, both of which are prescribed in sharee’ah:
1 – Praying the three rak’ahs continually with one tashahhud.
2 – Saying salaam after two rak’ahs, then praying one rak’ah on its own.
Both of these are narrated in the Sunnah, so if a person does it one way sometimes and the other way sometimes, that is good.
…
It is permissible to say it with one salaam, but it should be with only one tashahhud and not two, because if he does it with two tashahhuds, it will be like Maghrib prayer, and the Prophet (peace and blessings of Allaah be upon him) forbade making it like Maghrib prayer.
Al-Sharh al-Mumti’, 4/14-16
For more information please see question no.26844 and 3452, where there is a lengthy and detailed discussion about qiyaam (night prayers) and Witr.