Start by giving each of your controls a Title. You can do this by clicking on your control in your document and then click Properties on the Developer ribbon. The first field on the Properties window is Title. Give each one of the controls you need to access a unique title. In the example below, I'm using MyText, MyDate, and MyDrop for the text, date, and drop-down controls, respectively.
Then you can access each control in VBA by using the SelectContentControlsByTitle() function. As long as you're using a unique title, this function will return a collection containing only a single control so we can just retrieve the first item from the collection (index (1)). Here's how this would look:
Dim strText As String, strDate As String, strDrop As String
strText = ThisDocument.SelectContentControlsByTitle("MyText")(1).Range.Text
strDate = ThisDocument.SelectContentControlsByTitle("MyDate")(1).Range.Text
strDrop = ThisDocument.SelectContentControlsByTitle("MyDrop")(1).Range.Text
The Range.Text ending is what grabs the current text/value from the control.
Now that you have your three pieces of information, you can concatenate them into one string using the concatenation operator (&):
Dim strFilename As String
strFilename = strText & "_" & Format(strDate, "ddmmyyyy") & "_" & strDrop & ".docx"
And, finally, save it:
ThisDocument.SaveAs strFilename