Browner isn't a LB. He's listed at 220, but goes about 210-215 and has been reported as low as 206 for the Hawks previously, although I'll admit that he could be putting on weight to nail somewhat of a more base defense role. Either way, as mean and nasty as he is, he's still really lanky and while a great hitter for a CB and very strong, he'll fail as an edge setter as a SLB on a regular basis. His being 6'4 absolutely does not make him as big as a LB.
Morgan and Pinkins will also likely fail at that aspect against run heavy teams as well and that's where you'll likely see Marsh get a shot. The position, as well as the formation that we're in in terms of 4-3 Under/Over or Nickel with Browner, will be largely a matchup based platoonage. Morgan and Pinkins are kind of redundant in that they are much the same player, but particularly early on, you're likely going to see a mix of 3 players. Whomever wins the Morgan Pinkins redundancy battle, Marsh against run heavy teams, and Browner against pass heavy teams.
Later in the season it will sort itself out. It will have to because we don't have the do all player who's as fast as Morgan and Pink in the 4.4s or bigger and more violent than Marsh. Irvin was a hell of a player.