The Warrior's Three are gods of Asgard, and frequent companions of Thor. They're not based in Nordic mythology like Thor, but drawn purely from the fellows you'd meet in medieval literature or old Hollywood's version of the Dark Ages. You have Fandral, the noble and dashing ladies man , Hogun the grim and fierce, and Volstagg the valiant and cheerful. They go on sturdy, manly adventures together against villains big and small, and undoubtedly do a lot of feasting, drinking, and wenching afterward. They're more than just thinly drawn supporting characters, and if they get a lot of screen time in the movie, they'll be a welcome balance to Thor's bombastic ways.
Cooper let slip that he was "approached" for the part of Fandral, though he stressed that nothing was signed and official. He strikes me as a little young for Fandral (he's a god, but he's seen and done some stuff), and not nearly dashing enough. But hey, one girl's "He seems nice" is another's suave and swashbuckling, so your opinion may very. He's certainly a good actor, which means more in the long run, and is such a reassuring glimpse into how seriously Kenneth Branagh is taking Thor.