Getting along with other dogs

Charlie has been here for about two weeks now, and every time we try to introduce him to our two grown male chihuahuas, my chihuahuas lunge at him. They just want to attack him. I know he hasn't been here very long but I just don't know what to do....what if they never accept him? Has anyone had experience with this kinda thing? What should I do? I'm having to keep him isolated in one room when nobody's home so my chihuahuas won't attack him! I just want them to be friends....