You can also use ItemSize -> Fit:
Panel[Grid[{{"left", "nextToLeft", "right"}},
ItemSize ->Fit,
Alignment -> {{Left, Left, Right}, Baseline},
Frame -> False, Spacings -> {0, 0}],
ImageSize -> Full]
Mixed settings like ItemSize -> {{Scaled[.1], 15, Fit}} also work:
Column[
Panel[Grid[{{"left", "nextToLeft", "right"}},
Alignment -> {{Left, Center, Right}, Baseline}, ItemSize -> #,
Frame -> False, Spacings -> {0, 0}],
ImageSize ->Full] & /@
{Fit, {{{15, 20, Fit}}}, {{{Fit, 20, Fit}}}, {{{Scaled[.1], 15, Fit}}}}]

Update: Need help locating references in the docs for ItemSize->Fit. I am quite sure I have seen it somewhere in the documentation, but could not find anything using the usual keywords.
The only example I currently have is burried in this great answer by Mike to a different question (thanks Mr.Wizard).
ItemSizewhen I searched with keywordsItemsize->Fit,FitorItemSizein the docs. – kglr May 15 '14 at 15:43Fithimself here: (7499) – Mr.Wizard May 15 '14 at 22:10Fit:) I had checked the reference pages forDockedCellsbut could not find any examples usingFitthere. – kglr May 15 '14 at 22:22