Need help with Merging cells but with separations

Hi I am a school teacher and we are using excel for some files for students, but are having difficulty.... hopefully this will make sense.... we are putting goals in a spread sheet for each child lets say in the d column...in a, b, and c we have marking periods 1, 2, and 3. in the first three columns we are putting letters in and i want to basically "lock" the a-d cells. we are also going to be inputting more in the d column so everytime we do that the letters in the a, b, and c column wont match up then with the goals in the d column.....I dont want to merge them because then it becomes one cell and i want a separation between the three.....can anyone help...hope it wasn't confusing

thanks in advance

Danielle