ok, first catch all the legendary cats/dogs (enteit raikou and you probably have suicune) then go back to tin tower (the place were you caught suicune) and talk to 1 of the monks (sorry cant remember which 1) and he will give you rainbow wing then some stairs will appear, ho-oh is at the top of tin tower. hes level 60 so be ready
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You can only catch Zapdos in Crystal by cheating. otherwise it is not in the game
you don't catch articuno in crystal but you can get one from red, blue or yellow.
catch it in the wild
you catch it at night
You cannot catch Mewtwo in Pokemon Crystal, but you can trade it from a Pokemon Red or Blue. Hope this helped :)