|The "formula|s needed for a dependent list" - there is only one formula required for a drop down in any given cell - is used in the Data Validation Source Field, not in a cell. The only way to see the formula is to open the Data Validation wizard.
Here are the steps to do what you want:
1 - Select the range of cells that contains the drop down choices that you want to see when A1 contains 1.
2 - Name the range List_1
3 - Select the range of cells that contains the drop down choices that you want to see when A1 contains 2.
4 - Name the range List_2
5 - Select the range of cells that contains the drop down choices that you want to see when A1 contains 3.
6 - Name the range List_3
7 - Select B1
8 - Choose Data...Validation...List
9 - In the Source Field enter =INDIRECT("List_" & A1)
If you don't yet have a value in A1, this may present an error when you click OK. Don't worry about, just click OK.
10 - Enter !, 2 or 3 in A1
11 - Check out the drop down in B1
What the INDIRECT function does is build a Name from the string List_ and the value in A1, giving you either List_1, List_2 or List_3.
The Data validation feature then uses the data in that Named Range to populate the Drop Down.
It is only because you asked to use 1, 2 and 3 that we have to "build" the Range Name inside the INDIRECT function since 1, 2 and 3 are not valid names for ranges.
If you wanted to use One, Two or Three, you would use the more simple =INDIRECT(A1) since One, Two and Three are valid names.
That, in a nutshell, is what is explained at the site that I provided the link to.
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