Maybe Zodiac was a mathematician?
Aside from victims’ SSNs, license plates, cab number and dates, his brain seems wired on occurrences of the number three in other ways as well.
The number three is a special kind of prime number, it is the only prime number that is divisible by three.
In math, there are sets of numbers called prime triplets.
One of the first prime triplet sets is {13, 17, 19}.
Zodiac used 13 symbols in his "My name is -" communication. (Z13)
He used 17 symbols in the Z340 rows.
He used 19 as a transposition cycle value in Z340. (Which creates the chess knight move effect)
That all these numbers related to the number three appearing together in his thought process does not seem random to me.
w/e regard to the knight’s move, one of the geometrical curiosities of the Templar Cross is that it illuminates prime number relationships.
Thank you for that hyperlink.
I was not aware of Peter Plichta and his work. He is in the same age group as Zodiac 
Although not related to Peter Plichta, one of the posters (Frebetta) on your link offers a John Baez page called Klein’s Quartic Curve. Seven graphics down, it has a clever heptagon image filled with a system of various multiples of the number three.
For those curious about threes, sounds like a pretty interesting read.
I am still thinking that Zodiac’s mind is obsessed with the number three, and of course relationships with foreign alphabets. I already went through Irish, Scottish and even Ogham, but now here is a look at Morse code.
All the Morse code letters that are made of three symbols are: D, G, K, O, R, S, U, W.
A simple anagram of these letters would produce family names like Kudrow, and what not, but we could perhaps try combinations of these letters as keys in the unsolved encrypted texts.
A thought:
Wonder if the police ever checked if any taxidriver, earlier that evening, drow a man from the Presidio to the area, Mason St /Geary St, where Zodiac later got into Paul Stines taxi?
Meaning… did Zodiac park his car in the Presidio area, got a taxi and went to the Mason St /Geary St area, went to some theater performance? Movie? Musical?, and afte that hailed Paul Stine’s cab, asked him to go to Maple St > Cherry St, Shot Stine and went back to his car.I’ve thought about that too TF, but think Zodiac would have considered LE might think of that as well. Still would be curious to know if LE did ask other cab drivers or bus drivers about that. I’d like to hope so.
There is a picture on Tom Voigt’s site of a bus: 
He says Zodiac could have caught a bus: “The most likely scenario is that Zodiac walked south about 7/10 of a mile to Geary, where he caught a bus.” (bottom right picture text)
I did wonder about the cab idea, but he could have called for a cab hours earlier and done any number of things, watch a play, watch a movie, visit a friend, and so on.
