Pokemon Crystal Ho-oh

You need to catch Suicune, Raikou and Entei before you can get Ho-oH. Once you have all of these pokemon, go back to Tin Tower, where you fought Suicune and talk to all the three Sages up there. One will give you a Rainbow Wing. Once you have it, a staircase will appear the top right corner of the room. Go down it and find your way to Ho-oH.