"Agnosticism" is a statement of epistemology. It merely states that there is no way to prove or disprove the existence of God. "Theism" and "atheism" are declarations of belief. The express an acceptance of an ontology.
You can be an agnostic theist, or an agnostic atheist. While the epistemological question of our knowledge of God may be inaccessible, it is difficult to remain completely neutral with respect to personal belief.
Do you think God exists? You are a theist. Do you think God most likely does not exist? You are an atheist. As it turns out, most atheists are agnostic, and are more than willing to admit there is a certain probability that some form of god exists. That probability is simply vanishingly small.
Atheism is not as un-scientific as being religious. In fact, as the lack of god is the null hypothesis, atheism is the default scientific proposition. Until there is a testable, positive hypothesis concerning the existence of god, theism is entirely in the realm of wish-fulfilling fantasy and philosophical self-gratification.
Agnosticism, meanwhile, is not a "third option." As you use it, as a state of superposition between theism and atheism, it isn't even coherent.