I also had three chicks who I tried sexing at about 1 week old. All three had white spots on their heads, two were a little more pronounced. They also all had the black splashing on their legs. I ended up with two roos and one hen.
I wasn't able to tell for certain until about 8 weeks or so. That was when the size of the cockerels took off and the pullet stayed small.
Good luck!