Question: Can I add an additional knot to the tzitzis (i.e., a triple knot instead of a double knot), since without this the last two knots often unravel?
Introduction: The minhag is to have five sets of double knots on each of the tzitzis, and four sets of loops in between them (Shulchan Aruch Siman 11 Se’if 14).
Although the Remo (ibid Se’if 3) advises to tie the ends of each of the individual tzitzis strings, the Mishna Berura (s.k. 18) quotes a dissenting opinion and concludes that it is preferable not to add an extra knot, unless there is a risk that the tzitzis strings will unravel without this. The issue appears to be “adding to the number of knots” (and the correct number has symbolic significance – see Shulchan Aruch ibid).
Furthermore, in this situation it could be argued that one is not technically adding a knot, rather expanding the existing double knot, which is not an issue of “adding to the number of knots” (since in the symbolic significance of the number of knots, the double knot is considered one knot).
Conclusion: Although it is certainly ideal to have the specified number of knots, if without adding an extra knot the other knots will unravel, it is preferable to add an extra knot.