The definition of agnostic is "A person who believes that nothing is known or can be known of the existence or nature of God or of anything beyond material phenomena." What the heck has that got to do with number 2? If I wrote agnostic in #2 and somebody consulted a dictionary you think they would understand what an atheist is!?
Note that I wrote "popular", not "technical" definition. In popular culture, to the average English speaking American, the word "agnostic" means "someone who does not know whether God exists or not," literally, someone who is on the fence about God's existence.
We are not lumping #2 and #3 together because they are different. We use two different names for them. #2 is "atheist" number 3 is "strong atheist".
This is not actually how most people advocating your position see it. They see the word "atheist" as encompassing every person who does not actively hold the belief that god's exist. You can then sort those people into different groups of weak and strong, but all are considered atheists.
Notice the extra word "strong"? That is because both an "atheist" and a "strong atheist" has an absence of belief in gods, therefore "atheist" but the "strong atheist" has in addition a "disbelief in gods", hence the term "strong". I don't find this confusing but logical and rational. Why do you find it confusing and why do you think people are that easily confused?
If there are 3 groups of people, why not have 3 names for them? Why is it so important to you to squish two groups together?
Also, I'm actually not a fan of the weak vs strong distinction, since it's not really hard or fast. I prefer the "agnostic" vs "gnostic" (technical sense, here) qualifier since that is much more descriptive.
An agnostic atheist is a person who believes that gods do not exist but does not claim to know this. They are not 100% certain about their belief.
The gnostic atheist is a person who claims to know that gods do not exist. They are 100% certain about their beliefs.
Much more clearly delineated, imo.