I don't think you can do it directly because you can't have a formula and a value in the same cell at the same time. You could have column B as the input column, and the corresponding formula in C (in C1 put =B1*3), which would give the result you need.
There are more complicated ways to achieve this, but this would be the simplest.
Another rather simple approach is to download the add-in ASAP-Utilities, which has the ability to take a range of cells and do any kind of arithmetic to those cells. But that is after the numbers have been entered. BTW, this is a valuable tool with over 300 useful Excel functions/abilities.
Or you could accomplish the same thing in Excel (2002) without that add-in. Put the number 3 in cell D1. Then after the numbers are entered into column B. Select D1 and copy. Then select the range of numbers in Column B, right-click and Paste Special - then click on "Multiply."