[duchy]

If you are used to sharing the childcare or your wife does more it probably feels incredibly daunting to think about taking three children solo but August is a long time away and by then you will be very accustomed to taking the kids out for the day and maybe even away for weekends solo even if they are living with your wife rather than a 50/50 arrangement. I think if it's still financially doable ( the first year of separation is very expensive with legal fees and child support as well as finding somewhere to live big enough for not just you but the children when they stay) then you'd probably regret cancelling by the time August comes around.
I'd stick the decision on the back burner for a while but ultimately many single parent families choose to holiday at Disney as it has everything for them.

With regard to time away from Mum, by then the kids will be used to staying overnight with you and you have time for even a couple of dry runs with weekends away ... plus they can always Skype with Mum whilst you are away.