Currently the only way is to cheat, but there is an event coming up soon hopefully.
The first legendary encounter is Reshiram or Zekrom depending on what version you have, then after you defeat N you can go catch Cobalion, virizion and Terrakion the legendary trio also after you defeat N you go to a certain place depending on which version you have they tell you about a roaming legendary called thundurus or landurus if you trade with a friend to get the other roaming legendary you can get tornados the third of the weather Pokemon There is 1 more, Kyurem the third and final member of the dragon trio alonside Reshiram and zekrom he is at giant chasm but he is a level 75 so bring appropriate pokeballs and Pokemon. the rest would be event only wi fi Pokemon like meolotta Keloudo and genesect hope it helped!