a tiger is not a generic phrase referring to all tigers in general. It means a single tiger, although it's not specific about which tiger. So statement "a" is true as a consequence of the truth of statements "b" and "c" -- any individual tiger is likely to be dangerous because tigers in general are dangerous.
The distinction in that case is subtle, and can often be ignored because of the correlation between the two statements. But in the second group of statements, the distinction is significant. "Extinction" is something that can only happen to a species as a group, not an individual member of the species. So statement "3" is not meaningful, because an individual tiger cannot become extinct. "1" and "2" refer to the entire species (genus, actually, but this isn't a taxonomy blog) of tigers, which is capable of extinction.
The tiger can be used both specifically and generically, depending on the context. If the preceding text has made it clear that there's only a single tiger that could be referred to, then it's specific to that tiger. For example:
There's a housecat and a tiger in the room. The tiger is dangerous.
In this case, it's referring to the specific tiger in the room.