If cells are empty hide rows

Try this amended code:

Sub Skjul_0_Projektaftale()

    Dim rowNum As Long, beginRow As Long, endRow As Long
    Dim CheckCol_1 As Long, CheckCol_2 As Long, CheckCol_3 As Long

    beginRow = 150 'first row'
    
    endRow = 180 'last row'
    
    CheckCol_1 = 4 'column number --> Gulvarme
    
    CheckCol_2 = 10 'First column
    
    CheckCol_3 = 16 'Second column
    
    For rowNum = beginRow To endRow
        If Cells(rowNum, CheckCol_1).Value <> "" Then
            Rows(rowNum).Resize(3).Hidden = True 'hide all 3 rows first
            
            If Cells(rowNum, CheckCol_2).Value <> "" Or Cells(rowNum, CheckCol_3).Value <> "" Then Rows(rowNum).Hidden = False 'first row has values, unhide
            
            If Cells(rowNum + 1, CheckCol_2).Value <> "" Or Cells(rowNum + 1, CheckCol_3).Value <> "" Then
                Rows(rowNum).Resize(2).Hidden = False 'second row has values, unhide first and second rows
            End If
            
            If Cells(rowNum + 2, CheckCol_2).Value <> "" Or Cells(rowNum + 2, CheckCol_3).Value <> "" Then
                Rows(rowNum).Hidden = False 'third row has values, unhide first and third rows
                Rows(rowNum + 2).Hidden = False
            End If
        End If
    Next rowNum
    
End Sub