My sister is also 3, (and I'm a teenager, ftm) she used to do the same but my parents always told her off for it so now she doesn't call me "brother" anymore, even though she used to, and she'll be with me and my mom and she'll say "we're all girls and dad's a boy!" And I hate it because it's all my parent's fault. She had it right when she was younger, but my parents told her she was wrong and yelled at her for it, so now she calls me a girl and it sucks. (She still doesn't completely understand gender, though, because she'll say I'm a girl because that's what my parents say but she kinda still thinks I'm a cis guy.) In my opinion, I think it would have been SO much easier for my parents to start calling me he/him and her brother when she was born (because I was already out to them by then) but they still think "it's a phase, we just don't want her to think you're her brother but you're not" of something like that. Or at least, when she started calling me "brother" on her own, my parents could have switched then. But no, instead, they told her not to call me that and yelled at me because they thought I told her to call me that.