2D Movement in a 3D Game

I am currently making a game with 2d and 3d sections in it and I am currently working on a part that should be in only 2d. However, I am not sure about how to make the player only able to move left and right, and NOT forwards and backwards even though the game is in 3d. I am currently just using the unity third person prefab as a placeholder. So basically I am asking how do I make a player that can only move left and right, and make a camera that will follow.