请在下列程序代码的基础上按照要求设计该程序, 完善程序中的划线部分。
Dim n As Integer, a1 As Integer, a2 As Integer, a3 As Integer
Function f(x) As Boolean
f = True
For i = 2 To Sqr(x)
If x Mod i = 0 Then ①
Next i
End Function
Private Sub Command1_Click()
n = ②
For a1 = 2 To n
For a2 = a1 To n
For a3 = a2 To n
If ③ And f(a1) And f(a2) And f(a3) Then
List1.AddItem (Str(a1) + " " + Str(a2) + " " + Str(a3))
End If
Next a3
Next a2
Next a1
End Sub
其中①处填入;②处填入;③处填入