I don't know who is in charge of these things, but the apostrophe in the answer blocks should be after the "o," not before the "clock." It stands in place of the "f" in "of."
Please send a bug report - https://support.duolingo.com/hc/en-us/articles/204728264-How-do-I-report-a-bug-
I wrote it is three o clock and the answer said there was a typo but gave my answer as the correct version. I understand that the absolut correct answer may have an apostrophe but in practice this is often omitted as in duolingo 's answer.
There are bugs in the system that are very difficult for the course writers to fix as computers interpret apostrophes in special ways.