Too early to say right now.
Judging on what I've seen from these players so far (not as much judging on their HC results), I think Muzza is by far the most talented player of these three. He's sort-of a chameleon - adapts his playing style very well to that of his opponents. Not sure if he manages to do this as well on clay court as he well as does on HC, though. I expect him to be a lot more of a force on the clay than he's been showing so far.
Simon is a very tough grinder-type of player, and yes, I can well see him do well on clay as it looks like the surface will suit his game.
As for DelPotro - have to say that his main strength lies within his serve, and usually that doesn't mean all too much against the REAL great clay court players (I know he's won a lot of tournaments on the surface, but face it - those weren't against the really good clay court-players around - Rafa, Fed, Davidenko, to name just a few) - and it's a know fact that a good serve tends to get neutralized on the surface.
I give the odds to Muzza right now, but for now we'll just have to wait and see what will happen!
Del Potro's main strength is his serve!?
Murray has beaten Simon on clay since then anyway - Hamburg this year.
Simon has improved a LOT since then, though.
One could argue that Murray has also improved a LOT.