Find sum of 3 that total a target from a List
In general it looks OK to me, but you could maybe consider the following:

1) Return a IEnumerable&lt;int[]&gt; instead of List&lt;List&lt;int&gt;&gt; and then yield the positive results when found: 

            ...
            else if (sumSoFar == sum)
            {
              yield return new int[] { iValue, jValue, kValue };
            }...


2) The names sumI, sumJ, sumK are somewhat misleading because they aren't sums. Better names would be valueI, -J, -K

  int valueI;
  int valueJ;
  int valueK;


3) For the fun of it, you could consider: Because you basically do the same same loop nested three times, it could be a candidate for a recursive function iterating over each addend and yielding all the positive sums. In that way you could generalize the algorithm to handle any number of addends...