Function iftrue(Fx, condition, show)
    iftrue = Application.WorksheetFunction.if(Fx & condition, show, Fx)
End Function

Function testsum(a, b)
    test = Application.WorksheetFunction.Sum(a, b)
End Function

Function testif(a, b, c)
    testif = Application.WorksheetFunction.if(a, b, c)
End Function

=Application.WorksheetFunction.Sum(Range("A1"),Range("A2")) = Range("A3").Value

Function IFTrue(fx, condition As String, show)
Dim tst As Boolean
Dim z As Integer
Dim t As String
'test whether the condition is assuming "="
If InStr("<>=", Left(condition, 1)) = 0 Then condition = "=" & condition
'Find whether there is one or two qulifiers
If InStr("<>=", Mid(condition, 2, 1)) > 0 Then z = 2 Else z = 1
'Parse string to remove qulifiers from the quotes and make everything a string
t = """" & fx & """" & Left(condition, z) & """" & Mid(condition, z + 1) & """"
'evaluate the formula string to resolve to True or False
tst = Application.Caller.Parent.Evaluate(t)

If tst Then
    IFTrue = show
Else
    IFTrue = fx
End If
End Function

=IFtrue(SUM(A1,A2),"=A3","Must Be True")