Option Explicit

Private Sub Command1_Click()

  If isInputBad(txtLoanAmt, txtIntRate, txtYears) Then
    MsgBox "Bad input"
  Else
    MsgBox "good input"
  End If
  
End Sub

Private Sub txtIntRate_KeyPress(KeyAscii As Integer)
    Call checkKeyStroke(txtIntRate, KeyAscii)
End Sub

Private Sub txtLoanAmt_KeyPress(KeyAscii As Integer)
    
    Call checkKeyStroke(txtLoanAmt, KeyAscii)
        
End Sub

Option Explicit

Public Sub checkKeyStroke(ByVal textB As TextBox, _
                            ByRef key As Integer)
        'MsgBox Asc(".")
    Select Case key
        Case Asc("0") To Asc("9")
            'do nothing, input ok
        Case Asc(".")
            If InStr(textB, ".") > 0 Then
                key = 0
            End If
        Case Asc(vbBack)
            'do nothing, backspace
        Case Else
            key = 0
    End Select

End Sub

Public Function isInputBad(ByVal t1 As TextBox, _
                          ByVal t2 As TextBox, _
                          ByVal t3 As TextBox) As Boolean
    Dim inputError As Boolean
    
    inputError = False
    
    Dim errorString As String
    
    errorString = ""
    
    If Val(t1) < 0 Then
        inputError = True
        errorString = t1.Tag & " must be positive"
    End If
    
    If Val(t2) < 0 Then
        inputError = True
        errorString = errorString & vbCrLf & _
                        t2.Tag & " must be positive"
    End If
    
    If Val(t3) < 0 Then
        inputError = True
        errorString = errorString & vbCrLf & _
                        t3.Tag & " must be positive"
    End If
    
    If inputError Then
        MsgBox errorString
    End If
    
    isInputBad = inputError
End Function