I don't think Loughborough ever had a third platform before the modern Ivanhoe service started, ie it was not 're-instated' - it previously only ever had platforms on the fasts. Open to correction though - I'm sure there has to be on MR expert in here who will know.
As a Loughborough resident I can confirm you're right, except that before the brand new Platform 3 opened today's slow lines were the goods lines, and didn't normally carry passenger traffic at all and the "fasts" were the passenger lines.