Auto fill results in a cell based on the products of two other cells

[dayoalani]

Hello,
Assuming in worksheet, column C is suppose to have 3 as constant in each cell.
How do I create a formula such that
(1) If I enter a number in any cell in column A, it will automatically multiply it with cell in column C and will automatically place the result in column D on the same row OR
If I enter enter a number in any cell in column B, it automatically multiply it with cell in column C and automatically place the result in column D on the same row.

I really appreciate answers from you.

[Palmetto]

In cell, say, D1: =C1*A1

Or

D1: = C1*B1

Both of the formula reference cells in columns A (or B) and C and multiply the contents, returning the result in column-D cells.

If the value of 3 is a constant then you could just use: =A1*3 or =B1*3

[dayoalani]

The issue is how do I make 3 a constant in column C such that when I enter a value in any cell in A or B, it will automatically fill the result of A1*3 or B1*3 in D1.

[Palmetto]

Simply type the number 3 into the cells in column-C. Because it is directly typed into the cell (as opposed to being the result of a calculation) it is a static value and therefore constant. If your range of cells is C1:C100, then enter the number three into all of the cells. (However, unless you intend to later change the constant to another number, you could avoid the use of column-C altogether and simply make the number a part of the formula as I stated previously.

What if both A1 and B1 cells are filled, which cell has precedence in the formula?
Cells in column cannot be automatically filled without the use of VBA, which I don't see a need for in this situation.

Enter the formula of =A1*C1 into D1, or use B1*C1 and copy the formula down the column. Cells in column-D will show a value of zero until entries are made in either A or B cells.