All holograms only have 2 dimensions. They "trick" us into thinking the image you are seeing in the hologram has 3 dimensions. The idea behind the Universe being a hologram is that everything exists in flat space; however, it appears to us as being in 3 dimensions. Think of a video game, where you are moving around in a 3d world, but it's not really 3d, it just "feels" like it is.