An embryo on day three should have 8 cells.  Ideally, each day we want our embryo to double in cell number.  We also want our embryo’s cells to be of equal size and have no fragmentation.   The better organized an embryo is, the higher likelihood of producing a lasting pregnancy.  Imagine that; an embryo who likes organization…. this (potential) baby is going to be just like its Mom!    😉