Using an IF function to refer to separate sheet

I am looking for a way use an IF function that refers to a column that is referring to a sheet.

EX:

NOTE: yellow box is what I want to have happen using the function

The ID column is populated from a separate sheet in my workbook Formula:

=Sheet2!$A$3:$A$48

=Sheet3!$A$3:$A$48

I want the Flag column to refer to the ID column from the other worksheet. In pseudocode: "If ID field references the Sheet2 Range, return value 1, ELSE, return 0".