The catch rates of Suicune, Entei, and Raikou is 3. Because "3" doesn't really mean anything significant, it may behoove you to read Bulbapedia's page on catch rates. If you're too lazy to do that, use this catch rate calculator. Remember to change the generation setting if you're not playing Gen III or IV!
Hope this helps!
Sources: This, this, and this.
