Is the third wheel 7th book of diary a wimpy kid?

No, "The Third Wheel" is not the 7th book in the Diary of a Wimpy Kid series.

Here's the correct order of the first 7 books:

1. Diary of a Wimpy Kid

2. Rodrick Rules

3. The Last Straw

4. Dog Days

5. The Ugly Truth

6. Cabin Fever

7. The Third Wheel

So, "The Third Wheel" is actually the 7th book in the series.