In a way this was asked before HERE
also was asked here again but I guess in a different somewhat way HERE
but that was a few years ago, anyway different ERA, lol
Legendaries respawn after beating the Elite Four (re-beating if you already have).
ALSO, Coballion can only be caught on monday, fridays & sundays...
Basically just keep trying, also try this youtube video
how to catch the 3 dogs