I would guess three is a cockerel but they're really too young. Wait until 5-6 weeks. That is when I could tell mine were most definitely boys. Here are a couple of my Marans cockerels at exactly four weeks. They don't redden up as quickly as many say (although some cockerels may) and I wouldn't say they're any easier to sex than most single combed breeds in my opinion. I held onto hope with a couple of mine since there are some Marans that redden up super early that perhaps mine could be pullets but just because some redden up early, doesn't mean all do.
Too early to tell. Some lines of marans have cockerels that mature really quickly, while other lines take longer to show. Unless you knew you had an early maturing line, I would wait until at least 6 weeks to guess sex.
Nothing screams cockerel yet, but you won't know for sure for a couple more weeks yet. I just had a 6.5 week old start to colour up - I was suspicious of him from the beginning, but he is just now showing signs of being a cockerel.
They're still pretty young.... For the record, both of the 2 BCM's I've had REALLY looked like Cockerals to me and turned out to be hens. Mine really did "red up," fast as well as cankles, they're pretty big ladies. I can hear them coming down the hall in my coop, stomp, stomp, lol.