Can someone explain to me what's the meaning of "as they come" /"as they came".

I find myself struggling with 3 sentences:

1. By the time she was young, Sue was as cool-headed as they came.

2. The old car might not look fancy, but it's as reliable as they come.

3. As leaders in the industry, their expertise is as good as they come.

Question

Can someone explain to me what's the meaning of "as they come" /"as they came".

I find myself struggling with 3 sentences:

1. By the time she was young, Sue was as cool-headed as they came.

2. The old car might not look fancy, but it's as reliable as they come.

3. As leaders in the industry, their expertise is as good as they come.

Answer

As [adjective] as they come/came = no one is/was more [adjective] than that person/thing.
"As they come" means, roughly, "as they are made/produced." So: Even though she was young, Sue was one of the least panicky people you could find. The old car might not look fancy, but you won't find a more reliable car. Because they are leaders in the industry, no one's expertise is better than theirs. 
Sue or the car or the industry leaders might not be the best at [adjective], but no one is better. 
I hope that helps!