I won't bother to candle until
at least day 7, day 10 even better.
Because, (1)You can damage embryos by excess handling when they first start to develop. They're more fragile then. This is especially true of shipped eggs.
(2) They don't need the temp flucuations they get when you keep taking them out of the 'bator.
(3) By day 10, you can see for certain any that didn't start at all, and it's easier to tell a true blood ring (dead embryo, no visible veining at all, just a ring) from funny looking veining that can fool you early on, causing many to pitch eggs that would have been fine
(4) Day 10 is plenty early enough to get rid of non-starters, they won't have had time to get really bad and explode or anything like that.
(5) Day 3, you often can't see much anyway, you just make yourself crazy with it.
Any I'm unsure of on day 10, I move to one end of the set, and make a mark on them. Then when I candle again on day 18, I pay extra close attention to those, and get rid of any that are not as they should be. I may or may not re-candle just the questionable ones at day 14 or 15, to see if there's any change. If I'm in doubt, I leave them in, but on the far side of the others, if I have room.