How to show MsgBox if a range of cells in a worksheet of a workbook is already comp.

Hi there,

I'm working on a workbook with 3 worksheets and one of the worksheets, worksheet1 has a range of cells that is looked-up to the other cells in the other worksheets, worksheet2 and worksheet3. can anyone give me a sample code that automatically prompts a MsgBox once all of the cells in the cell range in worksheet1 is completed?.
this is my code:

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but when i put this code in worksheet2 or worksheet3 and enter a data in either worksheets the MsgBox keeps popping out even when the range of cells on worksheet1 is not yet completed and even when the cell in worksheet2 or worksheet3 is not referenced in the cells of worksheet1 and everytime i enter a data, the MsgBox keeps popping out. Please Help

thanks in advance,