This article provides simple instructions on how to get Pokemon Beldum in Pokemon Emerald. It's the same in Ruby and Sapphire. It is very easy and Beldum is extremely valuable as it evolves into Metagross.
Steps
Step 1. Play the game as usual and complete it
It seems like a big step, but you need to finish the game to get Beldum.
Step 2. Fly to the city of Mossdeep
Step 3. There is a boy who, when you see him, will scream
"Over there is Stephen's house!" He may say something else, as well as give you a royal stone. If this happens, then you have already been to Stephen's house and you just need to return there to pick up Beldum. If this is not the case in your case, keep reading.
Step 4. Go to the top left house, this is Steven's house
Go inside and you will see the orb that contains Beldum, as well as a letter stating that Stephen left to find himself, coincidentally, in the Battle of Meteor Falls, Stephen specializes in steel and also has Metagross; what an irony! This is how you get Beldum at Emerald, Ruby and Sapphire.