On my last donor egg cycle, I had like 19 fertilised embies all doing well on day 3... by day five there all but 1 had arrested and that was was poor quality.
Similar stats on first DE cycle
In my circumstance, they explained it was likely DH's sperm was fragmented.
RE said that embies thrive for the first 3 days on the egg's DNA and then the sperms DNA starts to really kick in. With such a dramatic number of embies arresting on at the day 3 mark (two cycles in a row - with two different egg donors), it was "obvious" to them that the problem was with the sperm.
Best to you on your upcoming transfer. It only takes ones happy embryo!