prepositions - what does "I'll be back in 3 days" mean?

I'll be back in 3 days

Does it means

I'll be back after 3 days.

or

I'll be back within 3 days.

or

I'll be back 3 days later precisely.

Answer

I agree with Maulik's paraphrase: in three days generally means three days from now. However, I'd start from today and count three days on the calendar, assuming the person would return on that third day, not on the day after that third day.

I also think there's a little bit of wiggle room, depending on the statement and the context. For example, if an employee of mine says, "I'll have that report ready in three days," I'd interpret that to mean, "I'll have it done within three days, maybe even a little earlier." The word "approximately" is also often implied. If a renovation team says, "The renovation will be done in three months," then "give or take a few days" might be implied. Such variations don't seem as likely, though, when someone is talking about a return date that is only three days away.