R1C1 style references

I'm looking at the help file and it doesn't seem to elaborate enough.

It says that you put an "R" followed by a row number and a "C" followed by a column number. (A R1C1 is the same as a $A$1 in A1 reference style.) WHen I try to do this in the cell, Excel doesn't recognize the formula.

Other examples given are R[-2]C, R[2]C[2], R2C2, R[-1]. I can't get any of those to work either.

Now there may be a different way to do what I'm trying to do.

I'm trying to sum three cells and have the same relative reference if I move the cell that's summing of if one of the cell being summed gets moved. (I'm cutting and inserting rows and it seems that I have to go back and fix these summing cells to get the right numbers.)