Ed and Double D seemed to be closest of the three friends both of them have had their ups and downs with Eddy.

They're all the closest the others have. There is no closest friendship. Double D is kind and patient, so he can tolerate both Ed's filthy habits and slow mind, as well as Eddy's harsh defensiveness and own filthy habits. Ed is a big innocent ball of affection towards the others, and extremely forgiving; he doesn't even mind the abuse. Eddy may be the hardest to get along with, but it doesn't stop the other two for a second, and Eddy truly cares for them. He's just not so good at expressing affection, having not received much himself.

They're all willing to protect each other when it's down to the wire, too, clearly demonstrating a best friendship that is 100% equal. It's amazing, really.