Ideally, you'd only press Enter once. However, the basic problem is that you're not updating the bookmarks - you're simply using them as placeholders to insert the text. To update the bookmarks, you should be using code like:
Code:
Private Sub CommandButton1_Click()
Application.ScreenUpdating = False
Call UpdateBookmark("Snr", Me.TextBox1.Value)
Call UpdateBookmark("Bes", Me.TextBox2.Value)
Call UpdateBookmark("Ini", Me.TextBox3.Value)
Application.ScreenUpdating = True
UserForm1.Hide
End Sub
Sub UpdateBookmark(StrBkMk As String, StrTxt As String)
Dim BkMkRng As Range
With ActiveDocument
If .Bookmarks.Exists(StrBkMk) Then
Set BkMkRng = .Bookmarks(StrBkMk).Range
BkMkRng.Text = StrTxt
.Bookmarks.Add StrBkMk, BkMkRng
End If
End With
Set BkMkRng = Nothing
End Sub