I have changed my name since I was last registered

[NAL123]

1) I've had my car repaired since I bought it.

2) I have changed my name since I was last registered, I have enclosed a certified copy of an official document confirming the change...

Do they mean I have done those things at least once (or, exactly once?) since the time I bought it/I was last registered?

Can I say:

3) I've had my car repaired three times since I bought it.

 

[Tarheel]

The first one is certainly possible, but why you would need to say that I don't know.

The second one is fine.

The third one is fine.

 

[teechar]

1) I've had my car repaired since I bought it.

2) I have changed my name since I was last registered, I have enclosed a certified copy of an official document confirming the change...

Do they mean I have done those things at least once (or, exactly once?) since the time I bought it/I was last registered?

Context is important. They can mean either.
Note that #2 is a comma splice. Use a full stop instead of the comma in the middle, or add a conjunction (e.g. and) after that comma.

Can I say:
3) I've had my car repaired three times since I bought it.

Yes.

 

[Tarheel]

My apologies! (We've talked about comma splices a million times. I guess I didn't see the need for one more. )

If you don't specify a number there is no reason to assume a number. We just know it happened at least once. However, with the name change thing it's very unlikely that it happened more than once.