I'll actually say it depends...
K4 is the best of the 3 for pay, etc. but...SA has shiny new 777s. If you are looking to go somewhere and then move on a 777 type sure as hell beats a 747 type any day.
Also, as has been mentioned, when Connie kicks the bucket ALL bets are off. SA has other backing financially and I hear they are planning on getting more airframes but don't quote me on that.
And yes, K4 is in negotiations for a new contract. Does that make it better? No...short answer is the company is being even harder to deal with because we "are in negotiations". My bet is no new contract for the foreseeable future so I wouldn't put that in as one of your reasons to choose K4.
But gotta agree with Gringo...if these 3 are the best hope you have you might want to seriously consider a different career. Or at least Atlas...