excel, simple issue please help?

There are basically two ways of doing this.

1) Enter 100 in A1. In A2 enter =A1-3. With the mouse, left click and hold on the black square in the lower right of A2 and drag down the column as far as you want. Each cell in column A will be 3 less than the cell above it.

2) Enter 100 in A1. Click on the Home Tab, Editing Group, Fill, Series, click on Column, Type is Linear, Step value -3, Stop Value 0 or whatever value you want. Click OK. The series will automatically be filled for you. (you don't have to select the cells beforehand just have A1 selected first)

Type in 100 in the cell. Click on the cell. You will notice a black box on the bottom right hand corner of the cell. Click and drag that black box downwards to the number of cells that you want to fill in. Then, in the Home tab, click on Fill -> Series, and change the step value to -3.

Quick tutorial I made for you: https://www.youtube.com/watch?v=stgE7nJ11H0&featur...

Apologies for low quality, watch in 480p, fullscreen to see it more clearly.