I double checked with my installer and indeed Tesla themselves - no power wall 3 is in development ( not of course ignoring that could change )
There isn’t much cause for one here in the UK - you can usually only use 3.8kw as limited by the grid . So larger storage not really helpful . Even if pv panels came out tomorrow at 40% efficiency ( as opposed to 20% today ) the benefit is limited - charge your battery faster especially in summer .
As Tesla the only real improvements for a power wall 3 would be a physically smaller unit and of course a lower price .
So the biggest hurdle is actually the grid .