Is it true that "During/Over" can be used with present/past perfect continuous, but not "In"?

Question

I have heard that "During/Over/In" can be used interchangeably. However, both "During and Over" can be used with both the present perfect continuous and the past perfect continuous, but "In" is only works with the present perfect continuous. I am going to make up two examples below.

(1) During/Over/In the last three weeks, I have been looking for a cheap sports car. Finally, I have found a great deal.

(2) During/Over the last three weeks, I had been looking for a cheap sports car. Yesterday, I found a great deal.

Some people told me that "during, over, and in" refer to a period from a time three weeks ago until a recent time. That means you started looking for a cheap sports car three weeks ago and continued looking for one until recently. However, "in" extends the time period up to now. So, it cannot be used to relate to an event that may or may not end right at this moment.

My question is: Is it grammatically wrong to use "In" in my second sentence?

Answer 1

The phrase "the last three weeks" can be used with the present perfect:

During the last three weeks, I have been looking for a cheap sports car.

Since "the last three weeks" (just like that, with no additional words, such as "of December") refers to the present week and the two before it, the past perfect is not really an option, as the phrase includes the present as the terminus ad quem.

You might be able to mute the discord a little by adding a past reference time:

During the last three weeks, I had been looking for a cheap sports car, but when I woke up this morning I realized that I should be looking for a reliable sedan.

I'd still use have been looking there.

The same limitation applies to in the last three weeks.

In the last three weeks, I have been looking for a cheap sports car.

The past perfect is not licensed because the present moment is involved:

In the last three weeks, I had been looking^{ungrammatical} for a cheap sports car.