Integrating in general is hard, introducing a single chicken is even more challenging.
Whatever your separation method (dog crate, wire fence) put both food and water stations next to each other - one on each side of the barrier. That will get them used to eating together.
As already mentioned, they need lots of space, and time.
Free range time in the yard usually helps tremendously... there's lots of space to run away and food and water isn't necessarily close by.
Inside the run or coop, add extra perches or obstacles to break the line of sight of an offending chaser. Multiple food and water stations are necessary, this time far apart. Pecking order is going to happen no matter what. But if you notice that one bird in particular (Wendy or Gertrude) is being extra aggressive, put her in the crate instead and let the other 2 work it out for a few days on an even playing field.
Another popular method was mentioned earlier, though not my favorite: just put Cheese on the roost at night, nobody will be fighting in the dark. Check first thing in the morning to make sure they're all ok, some pecking and chasing is normal, but if blood is being drawn, try crating the main aggressor or free ranging for a while.
Remember, chickens will be chickens, and pecking order isn't pretty, but it's normal.
Best wishes!