I do agree with Ottakee, as well as AppyT, but it all depends on the horses.
I just had to deal with this situation recently. Adding two horses to a herd of 3. Here's what I did:
Charity was first. She was 10 months old at the time and very much submissive to every living creature. Almost fearfully so. Because of this, we turned her out by herself, then slowly let out the other 3 horses, starting with the one lowest on the totem pole (Dreamer) then once they ran around and hashed everything out and everybody went back to hanging out, I turned out the next highest (Rain) and again, they ran around, hashed everything out, then stopped. Finally the Highest came out (Joey) and he did the same thing, then they all ran around together, then stopped. If I were to turn her 10 month old 13.0 hh self loose on the ENTIRE herd of horses, she would have got thrown through the fence.
Miakoda was the last to join the herd. Miakoda would charge over the fence at the other horses while stalled (i keep them in pipe corral too) and I could tell she was going to be rank, mean, and nasty to the other horses. So, I let her figure out the pasture barriers (walked her to show her the fence) let her realize it was hotwire, then pulled her out and turned everyone out (including poor little Charity) Then I loosed her on the entire herd and let her fend for herself. She is 6 years old and 15.0 hh with a rank attitude. She needed the power of the entire herd to humble her all at one time. She was knocked all the way down to the 4th rank (Poor Charity... she is still #5!) but after 3 days, she bumped herself up to #2 underneith Joey.
Everytime a new horse gets added to the pasture, the heirarchy needs a few days to adjust, so the pasture can sometimes be semi-chaotic for those days.
I hope I didnt over-engineer it for ya! Welcome to the forum hun!