1) Abortion is the killing of an innocent person.
2) It is not morally permissible to use force against a doctor who is about to perform an abortion.
3) It is morally permissible to use force against someone who is about to wrongfully kill an innocent person.
I don't think the argument works, however, because it's very difficult to create inconsistent sets relying only on moral permissibility, because moral permissibility is both very weak and very description-relative. In other words, a particular action or type of action may be morally permissible and not morally permissible under different descriptions. What the triad really says is that while it is morally permissible to use force in cases answering to the description "someone is about to wrongfully kill an innocent person" insofar as that description applies, it is not in cases answering to the description "a doctor is about to perform an abortion" insofar as that description applies. And we should all be thankful it is so, because if this were not the case we would constantly be running into paradoxes of moral permissibility.
To create an inconsistency you need instead of (3) something like the following:
(3') It is never morally permissible to refrain from using force against someone who is about to wrongfully kill an innocent person.
And that is a very strong claim indeed.
