Or is there just no demand for 3 bedrooms in the core because they are too expensive for families?
I just having trouble believing that it's an issue of people not wanting to live in 3 bedroom units in the core, and more an issue of not being able to.
Especially when you see the issue happening in Toronto and Vancouver. Condos are the only type of housing that most families can afford, and yet there are barely any 3 beds being built there either. People in those cities likely aren't buying 2 beds because they only want one kid, but because they can't afford the space for more.
I suspect similar forces play a role in Edmonton. If you want a bigger family, it's cheaper to go to the burbs and buy a TH than it is to buy a 3 bed "luxury" condo. Edmonton just has that option whereas people in those other cities don't really, unless you're moving 1.5hrs+ away from the core.