How should I say this hour in words: "03:01:00"?
It's three and one minute o'clock.
It's three and one minute.
Or the word "o'clock" can be said only in hours with two zeros (1:00, 2:00, 3:00)?
You should say:
It is one minute past three (o-clock) in the morning.
or
It's one minute past three a.m.
But lots of people would say informally. O-three-O-one.
Dropping "minute" is common aswell:
I arrived at one past three
Or inversely for 2:59 :
I arrived at one till three
Without an "AM"/"PM" or other context people will assume you mean 3:01 PM
Are time-telling telephone services still a thing? If not, back when they were, the voice would say “At the tone, the time will be three O one, exactly. Beep At the tone, the time will be three O one and ten seconds. Beep”