He was locked up for three days means that for a period of three consecutive days at some point in the past, he was locked up. As for the second part of your question: Two consecutive days would be a day that comes right after another.
For people working five days a week, saturday and sunday are usually their days off. Mary had stayed in this hotel for five straight days. Two consecutive days would be a day that comes right after another.
For example, monday, tuesday, and wednesday are consecutive days of the week. In a row also means the same thing when talking. For example, monday and tuesday would be consecutive, as well as thursday to friday, etc. What are two consecutive days in a week?
In this case, the use of in is not possible. The version with for six days throughout december is deviant. In normal conversation i would use the three straight hours version as that is a bit more informal. Consecutive means one thing following another in a predetermined order.
Monday, thursday, and sunday are not consecutive. Mary had stayed in this hotel for five consecutive days. Are the two phrases equivalent?