As an example, say we have numbers 1, 2, 3. Then, we find the sum of the three numbers, which is six. Then, we take the numbers and lay them out in a row. Then, we ask the sum of two slips. Then, we ask for the sum of another two slips. The one with the greater sum has the biggest number in the group. The one with the smaller sum would have the smaller number in its group. Then, to know which one is the middle number, we take the value of both groups minus away six, and you get the middle number.
Take another example. This time, we have the numbers 1, 2, 3 and 4. We ask the sum of two numbers twice (with one including in both sums), and once again, the one with the greatest sum contains the greatest number and vice versa. This time, though, as long as one sum is greater than 6, you know that 4 is included in one of the sums. If so, you know that the number that was added together with 4 (the group with the greatest sum) is either 2 or 3. If the sum is 6, 2 was added with four. If the answer is seven, 3 was added with four. If the other sum is 6, then two was added with four, or three with four if the answer is seven. The number that was not included is the remaining number that wan't touched at all.
Thus, as @eyl327 stated:
Five/Six Numbers would take 3 questions,
Seven/Eight Numbers would take 4,
Nine/Ten Numbers would take 5.
And this would continue on and on