https://www.gamespot.com/articles/pokemon-go-update-out-now-heres-what-it-does-not-m/1100-6453890/

Like Pokemon Go's previous Legendary Pokemon, Entei, Raikou, and Suicune will appear as Raid Boss battles at Gyms. You'll need to defeat the powerful Pokemon first in order to get a chance to capture it. You can check out our guide on how to catch Legendary Pokemon for more tips and details.