OneTbb version:
$ time ./llil3vec-tbb big1.txt big2.txt big3.txt >f.tmp
llil3vec-tbb (fixed string length=6) start
use TBB
get_properties      CPU time : 3.30477 secs
emplace set sort    CPU time : 0.825981 secs
write stdout        CPU time : 0.866964 secs
total               CPU time : 4.99808 secs
total        wall clock time : 3 secs

real    0m2.890s
user    0m4.687s
sys     0m0.646s