This one won't quite work:
Generally speaking, this concerns tasks which outputs more tasks.
Here, "outputs" is a verb, not a noun (as in the other sentences). So when you strip away most of this sentence, leaving only the relationship between the word "tasks" and the verb that goes with "tasks", what you're left with is this:
"Tasks" is plural, but "outputs" is a singular verb. In this case, "outputs" should be "output", just so that it agrees with the subject.
The other two treat "outputs" as a noun:
Generally speaking, this concerns tasks whose outputs are more tasks.
Generally speaking, this concerns tasks whose output is more tasks.
Should you use a singular noun and verb, or should you use a plural noun or verb? This really depends on the noun in question; there doesn't seem to be a very general rule for it.
As for "output" though, you could really go either way in this case. It depends on whether you're thinking of all the little "outputs" as a single, collective whole, or whether you're thinking of them as individual units. If you're thinking of them as all being grouped together as a single whole, "output is" is more appropriate. But if you're wanting to emphasize the individuality of the outputs - that they're all a bunch of different, individual things - "outputs are" is more appropriate. It's really up to you in this case.
I'll also briefly mention that "outputs are" is something that you're more likely to hear around technical people than around the general crowd. Non-technical people would generally just go with "output is".