Show UserForm On No Cell Value In Hidden Sheet

I have a Workbook with multiple worksheets. On one of my worksheets (Sheet4) I have three cells, let's call call them A1, A2 and A3.

A1 = Annual Budget
A2 = Money carried over from last year
A3 = Current financial year.

Formulas in other sheets are dependent upon these cells. I also have 3 userforms, one relating to each cell, with each form containing a textbox which enters its contents into its respective cell after clicking a command button. The userforms should display if the cell it relates to is empty. For example,

Private Sub Worksheet_Calculate()
If .Range("A1) = "", Then
Budget_Form.show
End If
End Sub

The problem I have is that the userforms only appear if I manually change the contents of the cells that they relate to. I need the sub to run every time the workbook recalculates. On top of that, I want the sheet to be hidden but the subs to still run. After looking around on other sites, I have pieced together the following code and placed it in the code for Sheet4

Private Sub Worksheet_Calculate()..................