who

[Olympian]

All three are fine. The first is the least formal, the third the most. You are unlikely to hear the third very much in normal conversation.

@5jj, thank you. I could be wrong since I don't know about teachers here today, but when I was in school the teachers would insist on using 'whom' in written exams. My
impression was 'who' is used in AmE which we rarely read/heard at that time. Perhaps it can be considered archaic in BrE, but we seem to have several of those (archaic) in use here.

[5jj]

There are still speakers of BrE who insist that 'whom' is the only correct form in writing. I accept this for formal writing - indeed, I use it myself in formal writng - but it seems a little stilted in less formal wriitng and in all but the most formal speech, in my opinion.

I don't know enough about Indian English to comment on what is considered correct in that variety.

[Olympian]

There are still speakers of BrE who insist that 'whom' is the only correct form in writing. I accept this for formal writing - indeed, I use it myself in formal writng - but it seems a little stilted in less formal wriitng and in all but the most formal speech, in my opinion.

I don't know enough about Indian English to comment on what is considered correct in that variety.

@5jj, we use words/phrases like 'we will take (your) leave now'. Young people don't use it anymore I think. But when I went to America many years ago, they actually laughed when they heard this phrase. That is an example of archaic phrases in use.