Text Validation

If you're using the InputBox function, what you see is what you get; there's no way to validate input. If you're using a standard textbox, you can use the Change event to delete unwanted characters:

Private Sub txtBox_Change()

Dim pos As Integer
Static bIgnore As Boolean

' Don't run this code if Change event
' was triggered by our changes
If Not bIgnore Then
bIgnore = True
With txtBox
' Save cursor position
pos = .SelStart
' Replace invalid characters
.Text = RegExReplace(.Text, "[^A-Z0-9]", vbNullString, True)
' Reset cursor position
.SelStart = pos
End With
Else
bIgnore = False
End If

End Sub

' Requires reference to Microsoft VBScript Regular Expressions library
Function RegExReplace(ByVal strOriginalString As String, ByVal strPattern As String, _
ByVal strReplacement As String, ByVal bIgnoreCase As Boolean)

Dim objRegExp As RegExp
Set objRegExp = New RegExp
With objRegExp
.Pattern = strPattern
.IgnoreCase = bIgnoreCase
.Global = True
RegExReplace = .Replace(strOriginalString, strReplacement)
End With

End Function

You can add code to the textbox's Validate event to ensure that the length of the .Text property is at least 3 (Len(txtBox.Text) >= 3).