All three of your rifles are capable of killing an antelope at 600 yards (although I would want to runt he ballistics on the grendal to be sure). Knowing that, it comes down to which rifle are you most confident with? that is the one to take.
As an example, this past October a buddy and me went to Wyoming for our first antelope hunt. I had intended to take two rifles:
My 260 by MCR
My 280 - Updated!
As a result of some packing issues, I was forced to take only one rifle. While I wasn't happy about not having a back up rifle with me, I made the choice based on confidence. That meant taking the 260 as I had been shooting it all summer and fall in various shooting scenarios. It served me well...
2 VA LRHers in eastern WY
If it were me, based on cartridge alone, I would go with the 6.5x284 as it will send a heavier/better BC bullet faster and farther than the other two. The wind is ALWAYS blowing where the antelope play!
That said though, it's really all about confidence. take the rifle that if an antelope steps out at 600 yards in a 12mph wind, you would be surprised if you missed!