What does "left for dead" mean?

Literally means to leave behind to die; to abandon someone as being dead. (The abandoned person may actually be alive.)

In general use, it means to abandon something because there is no hope for it.

It means to me that they were left behind because everyone assumed they were dead or as good as dead.

Colloquially it also means to beat somebody in a race so comprehensively that they might as well have been dead

All three of OP's suggested interpretations of left for dead may be valid, according to context, although the third would more likely be phrased as left to die. In the real world I think it makes little difference which meaning one assumes.

@Martin Beckett correctly points up another, rather different, colloquial usage.

Per my comment against the question, Green Day's Live and let die (itself just an echo of the title of the Fleming book/Bond film) is unrelated.