Sum the Last 3 Values
The following formulas return a sum of the last 3 values in a range of cells. Note that the formula in the first example is based on data that does not contain blank cells, whereas the formula in the second example is based on data that does contains blank cells.
Without Blank Cells in the Data
=SUM(INDEX(A2:A10,MATCH(9.99999999999999E+307,A2:A10)-3+1):A10)
Based on the sample data, the formula returns 200.
Sample Workbook: Download
With Blank Cells in the Data
=SUM(INDEX(A2:A10,LARGE(IF(A2:A10<>"",ROW(A2:A10)-ROW(A2)+1),3)):A10)
Note that the formula needs to be confirmed with CONTROL+SHIFT+ENTER. If done correctly, Excel will automatically place curly braces {...} around the formula.
Based on the sample data, the formula returns 225.
Sample Workbook: Download