I think for the 3rd sentence, you don't need to use"其中“， because the subject is "联系老同学聚会”. While like in the 1st sentence, the subject is "很多人” lots of people, so you could use "其中“ to refer to among those people....So I think for the 3rd sentence, if you wanna use "其中”， it could be " 虽然老同学都觉得聚会比较麻烦，但是其中还是有少部分人喜欢聚会“ and the subject is "老同学”, and then, "其中“ means among those old classmates.