I don't know where they could possibly expand the T3 club. There's no more space up there.
The T3 club expansion was scheduled to open summer 2018. Now the website says "2018." I have not seen any other details than that, but it has been advertised that they would expand it since DL first took control of the lounge. There is a large wall to the left of the club entrance that has the sign advertising this. The wall appears to be covering another space, but I have no idea what's back there or what was back there. I presume offices or storage of some kind.
You can see this at
this link. Scroll halfway down to the box about Sky Clubs where it reads "Locations are now open in Terminals 2 and 3, on the Mezzanine Level, with showers at Terminal 2 so you can refresh after a long trip.
Look forward to an expanded Club in Terminal 3 in 2018, and a new state-of-the-art Delta Sky Club, accessible from both terminals in early 2021. "