What does it mean to say you are doing something "over the next 3 weeks"? [closed]

It usually means the period of time during which some activity will occur. It does not imply that you will be 100% engaged with the activity for this period:

"I will prepare the draft text over the next three weeks"

If it is a definite span of time, you would use "for the next three weeks"

"The road will be closed for repairs for the next three weeks."

"I will not be available for consultation for the next three weeks."

It means that you will be doing that thing during, throughout, or through the next three weeks, not necessarily continuously.

For example, if you said, "I am going to complete the model over the next three weeks," you might work on the model during class each day for the next three weeks, completing the model at the end of this time period.