insert formula based on cell entry using vba

i'm sure this can be done but i cant get an angle on the method. i want to use VBA to put a formula into cell G3 based on the users entry in cell D3 so, for example the user enters M in cell D3 and the VBA code puts the formula "if D3="M",A3,0"into cell G3

i know i could use a formula in the cell but i want it to work with multiple entries so i figure VBA is the way to go

if i could get an idea what to start with i could probably figure out the rest but i'm stuck. any help appreciated

thanks