英语定语从句有一些定语从句我不是很懂,我把答案写下来,望求各位给我说一下意思,I have three brothers

英语定语从句
有一些定语从句我不是很懂,我把答案写下来,望求各位给我说一下意思,
I have three brothers ；and one of them is a doctor.
变I have three brothers ,one of whom is a doctor.为什么是one of whom
2.We are living in an age __when____many thigs are done by computer.
3Jack,with whom I went to the concert ,enjoyed it very much.为什么是with whom?