We got 3 new Cuckoo Marans chicks last week to add to our flock and cannot tell what the sex is. We checked the wing feathers and the spot on the head, but have no idea. We are not allowed roosters in our town, so here is a pic.
Some say the spot on the head, can determine the sex.
With barred birds the chicks head spots are fairly reliable to sex them. A larger, splotchier dot are males and crisper smaller dots are the females.
A quick glance at your photo two have sharp small head spots and the third looks iffy, possible male. Take a look at the spots and determine or take better pic of that third chicks head to post it here. Other means to determine are legs size and how they start to feather in. If that third chick has the stockiest legs and in a week note the speed of wing feathering. If that third chick is last to (slower than others to) get wing feathers and largest legs then your confident it's a male.