You might also want to try to recruit one of the players to help you. If all three are resistant, pick the one that's at least willing to meet you half way. Talk to him aside for a bit and try to get him on board, or at least let him know that you're going to spotlight him in the next game.
Then, at a logical point in the session, turn to that player and try to engage him one on one. Have an NPC talk to him in character. Lead by example. Cut the other guy lots of slack and don't worry about how successful you are the first time out.
Later on, do it again. And again. And again. Bring in the other players during the session. The NPC starts talking to Buddy, and then turns to Joe. The NPC always speaks in character.
Baby steps. You want to lead them gently, not shove them into the deep end.
And, accept that this might not work. They may very well be happy with how the game is going and see no need to change. You need to show them that they can have MORE fun doing what you want. If it's not more fun, they won't do it. Why would they?
Remember, baby steps.