Legendaries Guide

Arceus - Event only

Action Replay:
52246C94 28038800
12247D8A 00004801
02247D8C E0021C39
12247BEC 00002001
02247D90 000001ED
D2000000 00000000

Articuno - 
Pokemon HeartGold / SoulSilver and FireRed/LeafGreen – First, you'll need to obtain all 16 badges to make the legendary birds appear. Articuno can be found in the depths of the Seafoam Islands which you will need Surf to reach. Make sure you've got a Pokemon with the HM move Strength, as you'll need to move some boulders around to reach Articuno. He's roughly level 50, so make sure to bring an appropriate team.

Pokemon Platinum – Once you get the National Dex, the three Legendary Birds (Articuno, Moltres and Zapdos) will be roaming Sinnoh in the same way that Cresselia and Mesprit jumped around from place to place. Use your Marking Map Poketch app to see their current locations and track them down. Once the encounter begins, use a Pokemon with the Mean Look move to prevent each bird from fleeing (or better yet, use a Wobbuffet – its Shadow Tag ability will automatically prevent the bird from fleeing, even if you switch it out) and you should have all three in no time.

Azelf - 

Pokemon Diamond / Pearl / Platinum – Once you’ve beaten Team Galactic at Mt. Coronet, the lake trio of legendaries (Azelf, Mesprit and Uxie) will become available. You’ll find Azelf at Lake Valor.
Celebi - Event Only

Action Replay Code: (L+R before going into Pokemart)

94000130 fcff0000
62111880 00000000
b2111880 00000000
e000f710 00000088
685c0014 462a0000
4e640300 3dcd98aa
e5015248 2c45a57e
ebadfb57 a259f359
97ee7d12 50622e14
9db56c71 f1fc5532
3a497c7d 6e52011a
c66c9b7a 9b76815d
373d2d40 8c545892
b553198b 0b0d2667
e2073d3f f46b6618
f83dc582 70e5862b
b095da94 89df3f1b
c8beec3c 00f02aa2
5ef4424d 2dba035f
60a585df df5d19b2
08d64730 334c5371
d2000000 00000000

Pokemon Diamond / Pearl / Platinum – After beating the Elite 4 and obtaining the National Dex, head to Canalave City. On the west side of the city by the boat dock is a house with a sleeping boy in it. Talk to the boy, then take the boat across to Fullmoon. Follow the path (there’s only one way you can go) until you reach Cresselia, who will immediately flee when you approach. Make sure to grab the Lunar Wing she leaves behind, and bring it back to the sleeping boy to cure him. Now you’ll be able to find Cresselia roaming around Sinnoh (use the Marking Map Pokeketch app to view her current location).