Well gender neutral clothing for a 3 year old in our culture would be anything commonly worn by boys or girls.
So t shirts, jumper, sweat shirts and jackets in bright or neutral colours.
Jeans, shorts, joggers, trainers, crocs, converse, wellies, ankle boots
In fact what many of us wore as children and what I mostly dressed my own children in.
And what many of us are wearing right this very second. I’m wearing a t short and joggers. Yesterday it was trousers and a navy top because I was at work.
Welcome to The world of most British women most of the time.