If IsNull(Forms![FORM_T3]![AGA2]) Then
Forms![FORM_T3]![AGA2] = Me.TARS4.Value
Forms![FORM_T3]![AGA3] = Forms![FORM_T3]![AGA1] - Forms![FORM_T3]![AGA2]

Else

If Me.TARS3 = "" Then
Forms![FORM_T3]![AGA2] = Me.TARS4.Value + Forms![FORM_T3]![AGA2]
Forms![FORM_T3]![AGA3] = Forms![FORM_T3]![AGA1] - Forms![FORM_T3]![AGA2]
 
ElseIf Me.TARS3 = "" Then
Forms![FORM_T3]![AGA2] = Me.TARS4.Value + Forms![FORM_T3]![AGA2].Value
Forms![FORM_T3]![AGA3] = Forms![FORM_T3]![AGA1] - Forms![FORM_T3]![AGA2]
 
End If
End If