I'm always on the look out for these - non-princess-related female lead books! DD is almost 3 and likes:
Katie Morag
James Mayhew's Katie books
Helen Stephens, How to Hide a Lion and How to Hide a Lion from Grandma
Meg and Mog
Witchety Sticks
Tiger Who Came to Tea
Emily Gravett, Matilda's Cat and Monkey and Me (a bit young for 3 though)
Rebecca Cobb, Lunchtime
Shirley Hughes' series of rhyme books with a girl toddler and her brother
Catherine Rayner, Abigail
Mrs Pepperpot
Polly Dunbar, Tilly books
Millie's Marvellous Hat
Books from children's TV series with a female lead - Charlie and Lola, Sarah and Duck
When DD's a little bit older I'll start with Milly Molly Mandy, My Naughty Little Sister and Olga da Polga.
It's definitely the case that most children's picture books have male leads - I love the illustrations to eg. Oliver Jeffers books but there are almost no female characters! (heart & the bottle excepted but that's not exactly cheerful for a 3 yo). Most books for that age follow the "boy and his favourite animal/soft toy/adventure/problem" format. Nothing wrong with that if there were plenty of other options out there, but it not only gets a bit samey but teaches girls that the default narrative is the male adventure narrative. I don't particularly like even those books which have quirky princesses as that still tends to reinforce the sense that the princess narrative is the dominant one for girls.
Try finding books where non-white children are the main leads though - even worse! :/