Rereading over the Supergirl and the LSH and I realize that Earth Prime has a Kandor. These Kandorians are native of Earth Prime, so does that mean we have a group of characters with powers of the Silver Age Superman?
This makes sense that Brainy would charge the 3-boot legion with protecting the multiverse from Superboy Prime. It's not that they're any more powerful than the other Legions, but they have a relationship with a group of Kryptonians who are 1) just as powerful as Superboy Prime and 2) 1,000 years more advanced, physiology-wise, than Superboy.
It's hard to say if that Kandor is not the Kandor of Earth-0, moved there to keep it safe from Brainiac or if it's even filled with genuine Krypton-Prime Kryptonians. This revelation is new since Waid had plans to talk about Kandor down the line and tie his Legion into the DCU, before the three Legions storyline came about.