I have a excel worksheet with the one column (9000 value) and i want to have every 3rd value of the column. Or is it possible to have a command which gives the average of every 3 value ?
You can use an array formula to loop through your range and pick out every third value. Something like this will work, assuming your values are in cells A1:A9000:
But after inserting the formula, press Ctrl+Shift+Enter rather than just Enter. This confirms it as an array formula and you will see in the formula bar that curly brackets have been placed around the formula - at which point it will look like this: