The period between Osiris' death and resurrection varies, depending on the myth. For example, as "the Osiris"/deceased in the Egyptian funerary texts, as well as the nightly sun, he dies and is resurrected on a daily basis.
The annual death-and-resurrection period, however, is commonly depicted as three days. One such citation is Reverend Dr. Alfred Bertholet, a theologian and professor at the University of Göttingen. In an article entitled “The Pre-Christian Belief in the Resurrection of the Body”, published in 1916 in The American Journal of Theology by the University of Chicago Press, Dr. Bertholet remarks: "Osiris was three days and three nights in the waters before he was restored to life again." Being ‘restored to life’ after death is resurrection, in my humble opinion!
OK, so some detractors might argue, despite the Rev. Dr. Bertholet's scholarly credentials (while conveniently ignoring also his status as a Christian Reverend!) where is the historical evidence to support his claim?
It's here (and has been for the last 2,000 years - hiding in plain sight!) The three-day period and resurrection is also recorded by first century historian Plutarch (39, 366D-E) as occurring on the 17th, 18th and 19th of the Egyptian month Athyr (Hathor). (Source: Plutarch, "Isis and Osiris", translated by Frank Cole Babbitt)
Look to the Bible for similar mythologies surrounding death and resurrection. Jonah Chapter 2 verse 1 where Jonah is in the "belly of a fish" for 3 days. This cannot be read as an account of a real historical event so one must interpret it metaphorically.
Just a thought.