I have to be honest, as a bookseller, I consider 3-5 the prime PB age for almost any picture book. Most parents are now seeking to get their 5-9 year olds reading and easy reader/chapter books are what you see them buy for their 5-9 year olds the most.
Popular books include Lady Bug Girl, Olivia, Llama Llama, Fancy Nancy, etc., which really are prime for 3-5. Lesser known, personal favorites include "Wink, the Ninja Who Wanted to Be Noticed", "Tea Rex", "Dangerously Ever After", "The Princess and the Bag of Frozen Peas", and Mark Teague's latest Three Little Pigs retelling (the wolf just has low blood sugar...) Among soooo many others--there really are a lot of great picture books out there.