Preposition "in" indicating time

Hello, everyone.
thank you in advance for helping me with this.

When used before time period, intuitionally, I think it means within the period.
but, in the dictionary, i also get

4. when (a period of time) has elapsed: come back in one year
3 describing the time at, after or within which something happens in the morning; I'll be back in a week.thus, I am confused.
when would "in" mean at(time point) or after or within?

Suppose it is 12 o'clock now.
If i hear someone say "I will be there in 3 hours, meet me there!"

I can't tell if he would arrived
within 3 hours: that means I should get there right now, and wait till 3 o'clock.
or after 3 hours: that means I can get there at 3 o'clock, and wait for him.
or may it just mean around 3 o'clock? I can get there at 2:30, wait till 3:30.