delaying incubation by 3 days should not affect hatch rates. Eggs can generally be left for 10 days without affecting hatch rates. It is even recommended to leave for a minimum of 24 hours after travelling through the post.
So the things I am wondering are, how did the last hatch go? Was is early, late, didn't make it? This will give us an idea of how your incubator did.
I will assume since you didn't mention changing any settings that the last hatch went perfectly yet this time more than half have died.
So if the last one did go well and this one isn't it would point towards the eggs being no good.
You probably never candled the eggs before placing them in the incubator but doing this before starting is really useful, you will know if an egg is starting with a big air cell or whether it happens during incubation.
If you had discovered that some eggs were only half full before starting you could have complained to the seller for selling you inferior eggs but like this it's a bit hard to know what has happened.
Maybe they got damaged in the post - again candling would have revealed this before starting.
Good luck with the last eggs, me personally I would be double checking and triple checking the temperatue and humidity to make sure it wasn't my incubator that was the cause.
Ps: on most incubators you can test the turning motor by pressing the + button but this might vary from different models and should be mentioned in the manual.
Either it works or it doesn't work. Just like a car either drives or it doesn't it's fairly easy to tell , either it turns or it doesn't turn.
You need to check
