[dkofoed]

I'm thinking sometime in the next 2-3 weeks I will likely need to put a 3rd medium box on my hives - they are currently 2 medium boxes tall, on previously undrawn plastic foundation.

My question is ... when I do add the super, should I bait it with a frame (or 2?) of brood? Or would a frame (or 2?) of stores/pollen be better?

 

[Zbee]

I assume you want that 3rd medium for brood. If so, move a couple frames of brood up into the new box (centered) to encourage brood build out. You can take the displaced frames from the new box (undrawn) and place them individually between drawn comb in the lower chamber to encourage those frame to be drawn out too.