"in" as it relates to time means "right after". "in 3 days" means after day 3 starts but usually before it ends. "within" is used when you want to say something is in the 3 days, not after. IE, it can be on day 1, day 2, or day 3. "something happens in 3 days" - something happens on day 3. "something happens within 3 days" - something happens on either day 1, day 2, or day 3. Idk why English is like this, where "in" means "after" when it relates to time but it is.

Within and in are prepositions. Ex. Doctors spread alcohol within the Hospital building. Where is my pencil? Your pencil is in your backpack.