First bird is definitely the best of the three. It's hard to be completely sure what to keep and what to cull at this age. I don't do a first cull on my birds until they're like 10 to 12 weeks old actually. Unless they hatch with the wrong markings/color for whatever color they're supposed to be, then they get marked then.
Cuckoo chick is a bit lacking in fluff. Like you said, and crest needs something to be desired. Cuckoo birds in general sometimes have lighter toe nails or even toes when young. It may go away as it gets older? The lighter beak tip is normal too and may also darken with age. It looks to maybe only have one barring (cuckoo) gene because it's so dark in color. Last bird being dominate white may always have the light patches on the skin. You're right about the body typing looking more like some other breed. Most likely again, as you said, from whatever breed was used to transfer the dominate white coloring? No way to determine what breed that might of been. Going to have to maybe work on the body typing I think more with this one's offspring? Really only time will tell because they're so young now.
Just curious, what effect does cuckoo (barring) have on fibromelanosis? The poster previously stated she felt the chick was female, so that would explain the single barring gene, as it is sex linked. What effect does dominant (not dominate) white have on fibromelanosis?