They don't have values there because they aren't on the pauling scale of electronegativity, as they don't form any compounds with other elements. However, argon and neon can technically form compounds with other elements; it is just extremely unlikely. For example there is a compound called Argon Fluorohydride, HArF, that can exist, but is only stable at around 27K (-246C). I don't think Helium can form any compounds other than with itself.
The other noble gases also can react with fluorine (As well as other elements), and make slightly more stable compounds. Hope that helps :)