What is the formula to find the sum of cells A1A2and A3?

I assume you mean the formula as used in a spreadsheet?

Place the formula in cell A4

=SUM(A1:A3)

or you can use =Sum(A1+A2+A3) - This is a loger way of doing it but for someone new to calculations in spreadsheet cells its easier to check and edit if you need to change a cell reference.