Murray has a good chance at RG if he recovers quickly and well enough this year, and Nadal underperforms as has become customary for him these past few years (would that be considered underperforming then?). AO is too far away.
Wawrinka has the tools to win at Wimbledon but his current game isn't suited for it. But if he improves his serve and slice, he can do it.
As of right now, Stan is more likely to do it first if they both somehow pull it off. In terms of who's more likely to have achieved it at the end of their respective careers though, I'd say Murray.
Overall though, I find tennis is too unpredictable to say definitively whether I believe one, both, or neither of them can or will do it. I don't know how everyone else seems to be so sure about these things.