An 'If Then' Formula

Here is my situation:

Cells A1 to A3 are numbered 1-3.
Cells A4 to A6 are empty.
Cells A7 to A9 are numbered 4-6 if a number higher than 1 is entered into Cell C1.

I need a formula to do the following:

If the number 4 is entered into Cell A4, Cells A7 to A9 should now display the numbers 5-7, but only if a number higher than 1 is entered into Cell C1.

If the number 4 is entered into Cell A4 and the number 5 is entered into Cell A5, Cells A7 to A9 should now display the numbers 6-8, but only if a number higher than 1 is entered into Cell C1.

If the number 4 is entered into Cell A4 and the number 5 is entered into Cell A5 and the number 6 is entered into Cell A6, Cells A7 to A9 should now display the numbers 7-9, but only if a number higher than 1 is entered into Cell C1.

Is there any formula that can do this? Any help would be greatly appreciated.

Thank you very much. Now I realized that I actually need a formula for alphabets instead of numbers -

Cells A1 to A3 contain the letters A-C.
Cells A4 to A6 are empty.
Cells A7 to A9 contain the letters D-F if a number higher than 1 is entered into Cell C1.

I need a formula to do the following:

If the letter D is entered into Cell A4, Cells A7 to A9 should now display the letters E-G, but only if a number higher than 1 is entered into Cell C1.

If the letter D is entered into Cell A4 and the letter E is entered into Cell A5, Cells A7 to A9 should now display the letters F-H, but only if a number higher than 1 is entered into Cell C1.

If the letter D is entered into Cell A4 and the letter E is entered into Cell A5 and the letter F is entered into Cell A6, Cells A7 to A9 should now display the letters G-I, but only if a number higher than 1 is entered into Cell C1.

Is there a formula for that?

That's works well - I'm just having a slight problem getting the formulas to work on my worksheet. I've attached it so you can see what I'm trying to do. I'd like to have column B on your sheet starting on column AL on my sheet. The green cell is your C1 cell, the red cells are your cells A4-A6, and the yellow cells are your cells A7 and up.