i wouldn't group the addends differently in a problem like 1+1+1, because, no matter which way you place the ones, you will always be adding 1+1+1.
i would group the addends differently in a problem like 3+5+7, because, in the first problem you would first add together 3+5 to get 8, and then you would add 8+7 to get 15. although, if you were to add 3+7 first, you would get 10. 10+5 is much easier to add than 8+7.