Here are the corrections for each sentence with mistakes:

3. **Can you help us?** We are having a meeting in the school hall **on** Friday at **two** o'clock.
- **Correction**: "our" should be "us"; "at four" should be "at two."

4. Peter has done his housework and then **watched** TV.
- **Correction**: "watched" is correct. (No change needed.)

5. Don’t forget to turn off the lights when **you leave** the classroom.
- **Correction**: "leave" should be "you leave."

6. Doing **fewer** drives is a way to save energy.
- **Correction**: "fewer driving" should be "fewer drives."

7. The children's room is full of stuff, so their mother wants them to tidy it up right now.
- **Correction**: "room" is fine; just add "it up."

8. We find eating junk food is unhealthy because it will make you gain weight quickly.
- **Correction**: "eating junk food is" should be "eating junk food."

9. Football is the only sport that I am interested in.
- **Correction**: "which" should be "that."

10. If I speak more confidently at the interview, they will offer me the job.
- **Correction**: No change needed.

11. Do you know how eager I am? I will give some of them to my grandparents.
- **Correction**: "am I?" should be "I am."

12. I enjoyed listening to traditional music and going dancing with my friends.
- **Correction**: "go dancing" should be "going dancing."

13. I didn't use to swim when I lived in the countryside.
- **Correction**: "go swimming" should be "swim"; "live" should be "lived."

14. There are fewer tornadoes in spring than in any other season.
- **Correction**: "less" should be "fewer"; "than" should be "than in."

15. Is he an actor or a singer? - An actor.
- **Correction**: "or" should replace "but" (not present in this case, just a suggestion for clarity).

These corrections improve grammatical accuracy and clarity.