You are correct that the preposition "over" can have multiple meanings, including "during" and "beyond." In the sentences you provided, the meaning of "over" is as follows: In sentence (1), "over" means "during" and indicates that the festival lasts for a duration of three days. In sentence (2), "over" means "beyond" and indicates that the business was built up for a period of ten years and the process continued even after that time. In sentence (3), "over" means "beyond" and indicates that the person is more than sixty years old. To distinguish between the meanings of "over" as "during" and "beyond," you need to look at the context and the time frame being referred to in the sentence. If the sentence is referring to a specific time period, then "over" likely means "during." However, if the sentence is referring to a period of time that has already passed or a time period that extends beyond a specific point, then "over" likely means "beyond." The preposition "beyond" generally means "on the other side of" or "further than," while "over" can indicate "across" or "covering a period of time." The choice between the two will depend on the specific context and what you are trying to convey.

All meanings derive from an Old English word that means "above". Closely related to that meaning, it also means "more than" or "on high". Hence "he's over sixty", "he's over six feet tall". From the original meaning, it came to mean "so as to cover a whole surface", and from that "through the whole extent, from beginning to end". Hence "over thee days" or "over a period of ten years". There is no magic formula you can use to decide which meaning is intended. If someone is talking to you and you don't understand, you just have to ask them to clarify. In #1, "over" can modify "spread" or it can modify "three". Being right next to both words, both interpretations are possible. So it could mean exactly three days, or it could mean at least three days. My intuition tells me it would be the former, but the latter is also possible and I would have to ask if I were confused. In #2, "over" can modify "period" or it can modify "ten years". Since it is closer (separated by fewer words) in the sentence to "period", most people will assume it modifies "period". If the speaker had wanted the other interpretation, he would have said "a period of over ten years." In #3, there is only one possible interpretation.

Over can mean a span of time/situation. It can mean “during.” One hint is if you see a unit of measure/time, that most means this meaning. Over can mean “beyond” or past a location. Most of the time, if you see a location or a set point in space or time, it means this meaning.

"over" here is the same as "for" when used with time. "Over" does not mean beyond in these two examples. The festival will not last "beyond" three days. beyond - more than