These three lines are called the Shemhamphorash, which means "the explicit name", which was originally a Tannaic term for the Tetragrammaton. There is no need to publish separate gematria for these verses, because the entirety of the three lines is computed as one sum.
In the Sefer Raziel HaMalakh, the 72-fold name is derived from Exodus 14:19–21, when it is read read boustrophedonically to produce 72 names of three letters. According to Kabbalist legends, the 72-fold name was used by Moses to cross the Red Sea, and that it could grant later holy men the power to cast out demons, heal the sick, prevent natural disasters, and even kill enemies.
I think this calculation was created to celebrate the symbolic birth of the nation of Israel. As we see from Genesis 2, the creation of Eve, who is traditionally believed to have been the first 'birth' occurs under the letter Tsade (90). The multiplication of the 90 a hundredfold symbolizes the expansion of the term to cover the birth of a people, rather than a single individual.
The idea behind reading the lines boustrophedonically may have come from a stray reference to a "reversal" of line twenty by Rashi, but this may have been a reference to the reversal of the cipher (i.e. the reversal cipher). Later writers naturally assumed that the Shemhamphorash should be read boustrophedonically to reveal 72 angelic names.
