Three days: originally Good Friday, Holy Saturday, Easter Sunday. Later and now, it is Holy Thursday, Good Friday, Easter Vigil and Sunday. This tends to diminish the Sunday.
It makes the essential statement of the Christian faith: “On the third day he rose again from the dead” (Creed).
The Passover was originally, long before Moses, the shepherd’s search for new pastureland for his flock, at the first full moon in spring. The Exodus from Egypt took place at the time of the shepherd’s Passover. The Jewish Passover is the search for the new pastureland, the Promised Land. The death and resurrection of Jesus took place at the time of the Jewish Passover. The Christian Passover is our passing over with Jesus, through living and dying with him, into the heavenly pastureland, our eternal life with God.
The shepherds sacrificed the first lambs born. The Jews redeemed their firstborn sons and sacrificed firstborn lambs. Jesus sacrificed himself as the lamb of the Passover.
The Passover of Jesus is one single mystery celebrated throughout the Triduum. The one mystery of deliverance (liberation, redemption) is celebrated in different ways, using different images. (On Good Friday: the sacrificial lamb. In the Easter Vigil and on Easter Sunday: the victor over death.)
The Passover is an annual memorial of the redemptive act of God that constituted his chosen people. It is not just a mental recalling of the event. The Jew believes that when he celebrates the Passover he is actually there at the original event of the Exodus. Likewise the Christian believes that when he celebrates the Easter Triduum he is actually there at the original event of the death and resurrection of Jesus. This is not the same as believing that the original event happens again - that Jesus dies and rises again.