Full Idea
Suggestions for a third value for truth are "indeterminate", or a scale running from "true", through "mostly true", "mainly true", "half true", "mainly false", "mostly false", to "false", or maybe even "0.56 true".
Gist of Idea
A third value for truth might be "indeterminate", or a point on a scale between 'true' and 'false'
Source
Paul O'Grady (Relativism [2002], Ch.2)
Book Reference
O'Grady,Paul: 'Relativism' [Acumen 2002], p.33
A Reaction
Anything on a sliding scale sounds wrong, as it seems to be paracitic on an underlying fixed idea of 'true'. "Indeterminate", though, seems just right for the truth of predictions ('sea-fight tomorrow').