Recently adopted another cat...will my 3 cats get along?

So just a few days ago, my sister and I adopted another cat...he's an 8 year-old Maine Coon (aka HUGE) who is SUPER sweet and we felt bad that he was having trouble getting adopted so we decided to take him in...we have 2 other cats, an 11 year-old and an 18 year-old (who's still healthy but a bit senile), both females. We brought the cat home knowing that obviously the cats wouldn't get along at first, but we were hoping they would all get used to each other in time. Our 2 females were scared of him at first, especially because he's sooo big...but now the tables are turned and the new kitty is TERRIFIED of the other 2 cats despite being twice their size, especially the 11 year-old. He has never hissed at any of them or tried to fight them, at first he just ignored them when they hissed or growled and even tried to be friendly and now he's always nervous...will they ever be friendlier to each other? I feel guilty bringing him to a house where he is so scared :(