=query({Sheet1!A1:C; Sheet2!A1:C; Sheet3!A1:C}, "where Col1 is not null", 0)
I wouldn’t use a script for this; the worksheet formulas are much faster, at least most of the time.
Make sure you use semicolons to separate the ranges. Semicolons are the End_Of_Row operator for array literals.
If you really want to use a script…
function combineSheets() {
var sApp = SpreadsheetApp.getActiveSpreadsheet();
var s1= sApp.getSheetByName("Sheet1");
var s2= sApp.getSheetByName("Sheet2");
var s3= sApp.getSheetByName("Sheet3");
var s4= sApp.getSheetByName("Sheet4");
// If Sheet4 doesn't exist you'll need to create it here.
var s1values = s1.getRange(1,1,s1.getLastRow(),3).getValues();
var s2values = s2.getRange(1,1,s2.getLastRow(),3).getValues();
var s3values = s3.getRange(1,1,s3.getLastRow(),3).getValues();
// Now, we can put out all together and stuff it in Sheet4
var s4values = [];
s4values = s1values.concat(s2values,s3values);
s4.getRange(1,1,s4values.length,3).setValues(s4values);
}