The question is where you want the competition for resources to take place. On multi grafted trees some of the grafts may grow faster due to their position or vigour. I have seen trees like this where one of the grafts fails but you are still getting fruit from the others.
By planting three trees in one hole you are putting the competition for nutrients and water at the root level. If one fails you still have the others.
These are solutions for gardeners with limited space who have the time and interest to work with trees that are higher maintenance.
- If you have lots of space plant one tree of each type at the usual spacing.
- If space is at a premium and you don't mind the cost of buying three plants then plant them all in the same planting hole.
- If you don't have space or enough money to buy three trees then try one multi graft.
Edit: The advantages of multi graft or multi stem apple trees is that the different types can pollinate each other. If you only have room for one tree or one clump and there are no other apple trees around this is real advantage.
Both types of grafted trees require more skill and time to prune. You have to ensure that all parts are getting enough light. A clump of different apple types could get a bit tricky to prune but nothing out of the ordinary. I don't think the yield of fruit or quality will be any different between the two types so it comes down to space available and price.