We have bought a new car this week.

[Alice Chu]

Please tell me the differences between the sentences below.
1. We have bought a new car this week.
2. We bought a new car this week.
3. We bought a new car last week.

https://learnenglish.britishcouncil.org/english-grammar-reference/present-perfect
We do not use the present perfect with adverbials which refer to a finished past time:
but we can use the present perfect with adverbials which refer to a time which is not yet finished:
1. We have just bought a new car last week.
2. We have bought a new car this week.

 

[teechar]

2. We have bought a new car last week.

That one is wrong, so let's get it out of the way.

3. We bought a new car last week.

That's okay and pretty straightforward. The buying occurred in the previous week.

1. We have bought a new car this week.
2. We bought a new car this week.

#2 is also pretty straightforward. The buying occurred sometime during the current week.

For #1, you'd need a reason (appropriate context) for using the present perfect. For example:
A: Tell me about all the exciting things you've done this week.
B: Well, we've booked our flight to Japan. We've wallpapered the upstairs. Oh, and we've bought a new car this week.

That might just about work! However, I would ordinarily use the past simple even in that last sentence in the above example.