I have 3 5day old ducklings and they are absolutely terrified of me.
I've only had experiences with older ducks and I guess I had this silly idea the ducklings would be all trusting

Any advice to ease their fear? Or know what sort of age they become less frightened?
P.s they some reason aren't scared of my cats
In my experience it has a lot to do with how much time you spend with them- as of now, do you sit with them and just let them grow used to you?
Both of my girls knew me the minute they hatched, and before even that- and I spend so much time keeping their trust- They've never been scared of me. I can lean over them, kiss them on their heads, pick em up, and they rarely even waddle away-
However they are a friendly breed and as I say, I've been there all along.
Now, they're familiar with other people too, but dislike others. If my family members reach for them, they're afraid, they only like them if they bring treats! Which is why I believe the key to bonding with them is time.
I would sit close to their food source, and simply read a book or scroll through an app on your phone, just pass the time until they're comfortable coming closer to you- and I would do this every day until they're confident, eventually taking food from your hand.
Do you ever bring them out for supervised swim time? At this age, they love having a go in the bath tub, a few inches of warm comfortable water, supervised and only for a few minutes. They will need drying gently after with a paper towel or towel so they don't get cold, and putting in the warmth of the brooder afterwards. Doing things like this may help them get used to you, even though it may be a challenge catching them at first!