Yeah, that's the tough part! Narrowing them down to three is better than nothing, though, right?
Someone else getting an animal and then not wanting to take care of it is all too familiar to me.

Unfortunately, I cannot let an animal go neglected if I have the ability to at least give them their basic needs, so every time that happens I end up saddled with a
new creature I didn't want.

Thankfully, I've been able to impress upon my family my disdain for such behavior, so it hasn't happened in a while.
I don't know if this is the same with ducklings so take this with a grain of salt, but recessive white chicken chicks can be deceptively dark-colored at hatch, anything from just a tinge of gray on their backs up to being a smoky gray color overall that still feathers out pure white.
Agree, I no longer will even consider buying the high production egg layer hybrids or strains in chickens. Their bodies just can't sustain that type of production.

I have a few Cochin bantam pullets who went into lay way earlier than their mothers did and I've been strongly considering calling them culls just because I don't want to breed for early egg laying like that.
Very much so! I'm trying to breed them toward the standard, which requires hatching as much as possible and narrowing it down to the very best. The hard part is the narrowing down.

I also keep getting ghosted when people message me inquiring about my extras, which is disheartening. I really don't want to process these babies, but I just don't have the room for all of them once the snow starts flying, so I have limited options there.
Not sure how young you're talking with them, but have they been on medicated chick starter by any chance? Weakness in the legs can be a sign of a B vitamin deficiency, which medicated starter can cause. You could try getting them a B-vitamin complex and see if that improves their leg strength.
Good vibes heading your way