For those who see this and are still interested:

if (i < 3), r at index i is copied from s, r[i] uses the defined tribonacci sequence.

Your code returns extra space at the beginning of the string. That's why it fails. For example, with your code nameShuffler('Richard Cheese') returns

" Cheese Richard"

Show us your attempted solution, we'll try to help.

One of common newbie mistakes is printing the result instead of returning it. Consider checking your solution for that.