Its more overall size but a lot of factors weigh in. Males are 2x the size of females once they're full grown they are huge your number 2 and 3 look like females so they won't get much bigger than they are now. Also the females feather in quicker than the males, the males have thicker legs and a different stance, generally females feet are a bit closer to eachother so you see them standing on their own feet sometimes. The biggest difference between males and females is the noise they make. Males make kind of a huffing hissing noise while females make more of a higher pitched coo like a chicken. Neither will ever be loud and quack like other ducks. The males don't produce any actual noise the only noise you hear is the hissing and huffing and the only thing making that sound is the air. Also once they start developing caruncles(the red stuff on their face) the males will generally have a lot more ot it than the girls.
I'm not sure on your #1 maybe if you uploaded a picture taken from the front it would be easier to tell but 2 and 3 definitely look like girls to me, Muscovy are the only ducks I have and all of mine are girls
Are 2 and 3 about the same size? or is one bigger than the other? They both looked like girls to me at first but now looking closer it looks like 2 might be a male and 3 a girl? But if they are the same size they are prolly both girls.
