The undersupply of 3 bedrooms is real, in many neighborhoods in my city you just can't find them. Anecdotally, the lack of 3-4 bedroom apartments has been the major driver of couples I know, who were aiming to start families and who would otherwise love to stay in my city, leaving the city for the suburbs. 3-4 bedroom units are also popular with younger folk, who are often happy to have roommates, and share amenities in exchange for lower rent.