Had the Zodiac Killer set aside three portions of shirt to send, it is highly unlikely he would want to be sitting on incriminating evidence for too long and after the murder of Paul Stine in Presidio Heights on October 11th 1969 he would mail his three portions of shirt with his next three correspondences. His first being the October 13th letter, just two days after the murder, his next letters were postmarked within one day of each other, so he sent the second portion with these and the final piece was mailed with the Melvin Belli Letter on December 20th 1969. What he didn't do however, was mail a piece of shirt with the Fairfield Letter on December 16th, four days before the Melvin Belli Letter, why, because it was a fake Zodiac letter, not mailed by the killer. This is not conclusive, but other features of the Fairfield Letter, suggest this correspondence was mailed by an impostor, who knew little about the mind of the real serial killer.
Anybody who was aware of the Lake Berryessa attack could have concocted the phrase 'The Bleeding Knife of Zodiac', and more importantly its failure to highlight 'by knife' was an opportunity missed from a killer now determined to link himself to the authenticity of his correspondence, by enclosing the swatches of Paul Stine's shirt. The Zodiac Killer had his ego dented when his first cipher was cracked in a matter of days by schoolteacher Donald Gene Harden and his wife, Bettye June Harden, two amateur sleuths. His response was to send us the still unsolved 340 Cipher, crafted either with no solution or a solution that went far beyond simple homophonic substitution. The Fairfield Letter however, is a retrograde step into mediocrity, when the supposed Zodiac Killer now reverts back to scribbling nine symbols, showing little effort, in the apparent guise of a small code and beginning his letter with "I just want to tell you this state is in trouble, I will go for the government life", more reminiscent of a political agenda, with bold claims of another potential 38 victims. The 'knife' wording does look similar to the writing on the card door at Lake Berryessa, but the template for these letters was freely available throughout his correspondence up to this point. But he fails to corroborate this letter with a swatch of Paul Stine's shirt, why, because the real Zodiac Killer was to contact authorities just four days later.
It is widely acknowledged that two pieces of Paul Stine's shirt were sent, firstly to the San Francisco Chronicle on October 13th 1969, just two days after his murder and secondly on December 20th 1969 to the home residence of prominent attorney Melvin Belli, but it was likely a third piece was sent along with the '340 Cipher and Dripping Pen Card' on November 8th 1969 or Bus Bomb Letter a day later, outlined in two articles written by the San Francisco Chronicle and mentioned in a Youtube documentary. Anyone can believe in mistakes, but the wording in the Chronicle is specific, to say the least and the mailing of three portions of Paul Stine's shirt makes perfect sense, from photographs of the shirt on the Zodiac Killer Facts site marked A, B and C.
Here is an excerpt from the FBI files (Zodiac 5 page 46/249), clearly showing three shirt pieces were mailed.