Tristate Button

Excellent! Use whatever font works. The important bit is that it is embedded in the PDF. Actually it's the Symbol font I was thinking about, not Wingdings.  The Symbol font is type1. Its always available to PDF viewers so it doesn't need to be embedded. The checkbox uses ZapfDingbats.  AdobePI provides the same symbols as ZapfDingbats.  The check is characer "4" and the X is character "5".  No need to use the unicode. So I'll be using Adobe Pi for the script. You'll need to substitute the character values for your font.

if(color.equal(event.target.textColor,color.black))
{
     event.target.textColor = color.green;
     event.target.buttonSetCaption("4");
}
else if(color.equal(event.target.textColor,color.green))
{
     event.target.textColor = color.red;
     event.target.buttonSetCaption("5");
}
else
{
     event.target.textColor = color.black;
     event.target.buttonSetCaption("");
}

So, this code creates a single 3-state button. It's gets even more complicated if you want to use the 3-state value for something, or to use it as a radio button.