If one prays tahajjud in thirteen rak’ahs, performing tashahhud and taslim after every 2 units, after 8 rak’ahs of tahajjud, can he perform 3 continuous rak’ahs as Witr, with one tashahhud and taslim at the end, to complete tahajjud?

Yes, praying 3 rak'ahs of Witr with one tashahhud and taslim at the end is correct.