The Longman Dictionary of Contemporary English explains that "in followed by a time period is more often used to talk about future events • He'll be here in a few minutes."
Can't we use "in" to talk about the durations of events that were already finished or completed?
Is the following expression? "We solved the problem in three hours."
Let me confirm one thing.
"We solved the problem in three hours." Doesn't this sometimes mean that we solved the problem during (within) three hours, depending on the context?
You could say that it means "during a three-hour time frame or less", but we would not express it in this way.
Dialog to illustrate "in three hours"
We have three hours to finish this department report? Do you think we can do it?
Yes, I think we can definitely get it done in three hours or less.
This is great. It's three o'clock, and we've finished the report. We got it done in two hours.
The requirement was to have it done "within three hours" or no more than a three-hour period.
I would not say that it's a good idea to use "during" this way, as in "We finished it during three hours".
You could say "We finished it during the afternoon".
We worked on it during the afternoon and got it done by 5:00.
That is what your respondents have said.
If you wanted to talk about the future, you'd have to say:
We will solve the problem within three hours. (The problem will be solved between now and three hours from now) OR
We will solve the problem in three hours. i) Same as above, or ii) We will start solving the problem in three hours time, or iii) It will take us three hours to solve the problem after we start (which could be any time).
1________2________3_______| - not past 3 hours; no more than 3 hours
___in three hours_____