Вот что нашла на английской вики:
"~Synchronicity~ 3rd Part
Hitoshizuku said on her blog that the third installment is already in progress and all fans have to wait a while until it is released.
Trivia
It was once claimed that "The series was cancelled in May of 2010" and "There will be no third installment". An incomplete video of the third installment was put up on the Nicovideo broadcast, but was soon taken down. Afterwords, this was denied by Hitoshizuku."
Надеюсь это правда и мы дождёмся последней части трилогии.