As far as triple-stranded DNA in humans is concerned, it is not likely to occur.
There is only one known living thing that has been found that displays some sort of triple-stranded DNA. From Wikipedia.org's article on
Triple-Stranded DNA:
Triple-stranded DNA was first described in 1957. It is known to occur in only one in vivo biological process: as an intermediate product during
the action of the E. coli recombination enzyme RecA. Its role in that process is not understood.
Other than that, all other mentions of 3-strand DNA have been in science fiction, such as the 2005 TV series
Threshold, which had a
shape-shifting lifeform that infected humans through some sort of audio signal. Wikipedia article on the TV series:
en.wikipedia.org...
It'd be interesting to know about this, since alien DNA may in theory be triple- or even quadruple-stranded, the latter being far more likely to be
of exobiological importance.