そもそもcombobox3は結果表示のコントロールだから、ここに余計なもの置くべきじゃないわ

Option Explicit
Private Sub ComboBox1_Change()
If ComboBox1.Value <> "" And ComboBox2.Value <> "" Then
AddItemToComboBox3
End If
End Sub
Private Sub ComboBox2_Change()
If ComboBox1.Value <> "" And ComboBox2.Value <> "" Then
AddItemToComboBox3
End If
End Sub
Sub AddItemToComboBox3()
Dim MyYear As Integer
Dim MyMonth As Integer
Dim LastDay As Integer
With UserForm1
.ComboBox3.Clear
MyYear = Val(Replace(ComboBox1.Value, "年", ""))
MyMonth = Val(Replace(ComboBox2.Value, "月", ""))
'来月１日の１日前から今月の対象月の終了日を算出する
LastDay = Day(DateSerial(MyYear, MyMonth + 1, 0))
'リストボックス３に今日の日付リストを入れる
Dim r As Integer
For r = 1 To LastDay
.ComboBox3.AddItem r & "日"
Next r
End With
End Sub