A person frequently performs only three rak'aat for tahajjud, making this his witr. What do you say about this?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

There is no harm in performing the Witr prayer with three continuous Rak'ahs without saying any Tasleem (concluding the prayer by saying Assalaamu 'Alaykum) between them. It is confirmed that 'Aa’ishah described the way the Prophet performed Witr saying: “He prayed three Rak'ahs without any interruption (i.e. any Tasleem or Tashahhud) in the middle [this means that he performed them continuous with one Tashahhud and one Tasleem only]." [Ahmad]

So, the three Rak'ahs are performed with one Tashahud at the last Rak'ah so that it will not be similar to the prayer of Maghrib. Indeed praying the Witr prayer like the Maghrib prayer is prohibited, as the Prophet said: "Do not perform the Witr prayer exactly in the same way you do the Maghrib prayer." 

This is of course if you meant by your question the ruling of performing Witr with three continuous Rak'ahs.

Finally, for more benefit about Tahajjud (optional night prayers), please refer to Fataawa 81337 and 83333, and refer to Fatwa 83303 about the time and number of Rak’ahs of Witr.

Allaah Knows best.