Case
Group of three students

There is a group of three students that make other students feel like they can't speak up or they will be made fun of, and other students do not speak up because those three are a constant distraction in class. How can a teacher change that environment so that these three students do not intimidate the rest of the class anymore?

Solution #1
I would have a conference with the students individually, then I would conference with the parents and an administrator. I would also place the students in different locations in the classroom. The locations would be in a way where none of the three students could see each other or entertain each other.

I feel that having a conference with each student individually is a good idea. This allows you to talk to them one-on-one and express your seriousness. If the problem persists getting the administration involved is, in my opinion, a good solution.

Talking with the students is the best option.

Solution #2
Separate them and crack down on the ring leader. There is always a student who leads a disruptive group. If you crack down on them with consequences, the others should follow.
Solution #3
It sounds like it is time for an affective domain unit. It is up to the teacher to foster a positive environment for all students, and this teacher may have missed the opportunity on the first day, or week of class. Regardless, all students need to understand the importance of respecting one another's beliefs, opinions, etc.
Solution #4
As was previously stated, I would first separate the students so that they can't make fun of other students in their small group/in whisper tones. Additionally, I would have a class discussion reminding students of appropriate behavior and ways to make others feel included. If neither of these things worked, I would start working with the most difficult/ring leader student of the group first and hope that cracking down on her would make the other two students stop. If it didn't, I would also conference with them individually. If none of those worked, I would try to have a guest speaker talk about bullying in the classroom.
Solution #5
Try a token system. Each student only gets 1 or 2 tokens which means they can only talk a certain amount of times. This would show that all of the students are equal.
Solution #6
Talking to those students may be helpful. I think that separating the students in the classroom will help because they will not be able to thrive off each other. Force them into groups with other students so they can make more friends and make everyone feel more comfortable in the classroom.
Solution #7
My solution would be simply to just split of the three of them and randomly place them in different groups.

I agree, I would encourage others to play and break up the threesome. Allowing more kids to engage in other activities together and to build more friendships.

I think that splitting them up would make class easier, but I don't think it would solve the problem. I feel these girls would still be together in out of the classroom situations like lunch or recess, and they would still feel inclined to behave in the same manner. I think more of a head-on approach should be taken.