Apparently my Best Buy decided not to do this. I went in before 8 last night and there was no E3 goodness to be found. I found an employee and was all "Yo bitch, there's a severe lack of E3 goodness and if I don't get any soon I'm going to go all sorts of crazy in this place" and then she told me they decided they didn't want to do it after all.
Meanwhile in Europe/Eastern Europe/Australia and Asia....
That's cool.
I don't know if the Best Buy near me does it but it definitely sounds like a good idea to me.
If they're doing it like how they did the one I went to, you'll only be able to play one of the games (or get lucky like me and play 2).
My advice? Try to go with Mario Kart 8. Windwaker is just a remake and the boss fight with the bird on the tower, Donkey Kong is coo', though a bit short, while the Mario 3D demo is quite short. Mario Kart was really fun to play and you get a good amount of gameplay from it.
Damn, I was really hoping for a SSB4 demo.
Also,