You can use a combo chart and add a second expression. On this second expression untick the bar, and line options but tick the Values on data point box. This will remove the line and bar but sit a number on top of your first expression bar.
You can then use an expression like the one below (this one will just put the value on the top 3 bars).....
if(match(rank(sum([Sales Qty])),1,2,3),sum([Sales Qty]))
You can incorporate something like if(rank(exp) > Max(rank(exp)-3)........ etc to get the bottom three values.