I am working on UI design for a new web based medical charting system. The users are used to charting on paper where they circle a symptom if it is present, cross it out if it is absent, and don't touch it at all if they don't address it. I need to replicate those three states in the UI. I don't want to use radio buttons since there is no conventional way to "clear" radio buttons, and three radio buttons per item feels cluttered. I put a few ideas below, but I'd appreciate any input.
Here is a jquery plugin that implements this design. Go easy on me, I'm new to web coding.
So here is how I decided to solve this problem, based on the answers from smurf and tim.baker below, modified for a mouse vs touch environment.
To keep the target areas from being too small, I made the entire half of the label a target. Here is a fiddle so you can play with it. Let me know what you think, I'm planning on making it into a jquery plugin.