var shift = 3 // mm
var matrixOdd = app.transformationMatrices.add();
matrixOdd = matrixOdd.translateMatrix (-shift/0.352778, 0)
var matrixEven = app.transformationMatrices.add();
matrixEven = matrixEven.translateMatrix (shift/0.352778, 0)
function main () {
    for ( i = 0; i < app.activeDocument.masterSpreads.length; i++ ) {
        mp = app.activeDocument.masterSpreads[i].pages;
        if (mp.length == 2) {
            mp[0].transform(CoordinateSpaces.INNER_COORDINATES, AnchorPoint.centerAnchor, matrixOdd);
            mp[1].transform(CoordinateSpaces.INNER_COORDINATES, AnchorPoint.centerAnchor, matrixEven);
        };
    };
    for ( i = 0; i < app.activeDocument.pages.length; i++ ) {
        p = app.activeDocument.pages[i];
        if (i%2 == 0) {
            p.transform(CoordinateSpaces.INNER_COORDINATES, AnchorPoint.centerAnchor, matrixEven);
        } else {
            p.transform(CoordinateSpaces.INNER_COORDINATES, AnchorPoint.centerAnchor, matrixOdd);
        };
    };
};
app.doScript(main, ScriptLanguage.JAVASCRIPT, [], UndoModes.ENTIRE_SCRIPT, 'Shift in the root');