4-week-old barred rock -- girl or boy?

OK, I'm throwing this out there to get past my fear of being told Sam is a boy!  I have three 4-week-old chicks (first-time chicken keeper) -- all different breeds.  I've got a Barred Rock, a Buff Orpington, and what was sold as an Ameraucana that I'm assuming is really an Easter Egger.

My understanding is that the BO is typically the largest of these three, and the EE the smallest.  Well, Margy, the BO, is the smallest of the three so far, and Francis, the EE is in the middle.  I'm pretty confident that those two are girls.  After several hours of reading and looking at pics here on BYC, I've gotten my hopes up that Sam, the Barred Rock, is also a girl, but I'm asking for your input to put my doubts to rest.

I'll attempt to post several pictures for you to make your judgements on....

and for good measure, here's a not-so-great shot of Margy and Francis:

Thanks for your help!  ~Garnet

Agh...  yes, I thought this was a tough one.  At first, because Sam was noticeably getting bigger than the other two, with a more developed comb, I started thinking she was a he.  But the more I looked at pictures and read about the differences here, I started getting more confidence that she is indeed a she.

Her (fingers-crossed) coloring seems lighter in the pictures than it does to me in real life.  Not sure why that is.  Also, I'm attaching another pic that shows the dark coloring on her legs.  The comb is a tad pinkish, which has me questioning.  All three are growing pretty fast, and Francis, the EE, already is getting to be about as big as Sam.  Sam's legs aren't much thicker than Francis' at this point.  Margy, the BO, on the other hand, is unquestionably smaller, and frankly she makes me worry that there's something wrong with her.  But, she bustles about and is eating and drinking, so I'm trying not to worry.

All three came from a local feed store on the same day, and were all supposedly hatched the day before I got them.  The feed store said there was a 10% chance of getting a male out of each of the 100 chicks of each breed they got in that day based on the uncertainty in determining sex.

So, here's another (partial) pic of Sam.  After this, I guess it's wait and see!  Thanks for all the replies so far!

Hi All!  Not sure if anyone will see this, but my chickies are 6 weeks old now, and I thought I'd throw up some new pics of Sam to see if anyone wants to add, confirm, or change their opinion about whether she's a pullet or a cockerel.  Based on Happy Chooks 6wk pic, I'd say Sam is definitely a pullet.  Still, her comb has more pink than my other two, and it looks like she's just starting to develop wattles, which I don't notice on the other two yet.

Anyhooo....   what do y'all think?  Thanks!