All three shiny legendaries can be downloaded at GameStop. Also, these are the ones that allow you to encounter Zoroark if traded to Black/White.
It is not HG/SS exclusive: Platinum, Diamond, or Pearl can download them as well.
Raikou will have Weather Ball, Entei will have Flare Blitz, and Suicune will have Sheer Cold.
Raikou can be downloaded during January 3rd-9th.
Entei can be downloaded during January 17th-23rd.
Suicune can be downloaded during January 31st-February 6th.