if you have only three spacers (and they are of the same dimensions) and can put them under the stem, you should be able to remove them from under the stem and put them on top too. or put one under and two on top. or two under and one on top. remember your stem is just another spacer as far as your steerer tube is concerned.

of course if when you have all the spacers under the stem and the gap from the top of the stem to the top of the steerer is more than one your aforementioned equally dimensioned spacers, well, then all bets are off.