To make this easier, we should give all the amounts like denominators (the same bottom number).
The denominators we have listed are 3,4, and 6. We can see that if you double 6, you get 12, which is also 4•3. The like denominator should be 12.
Now we have to make each portion equal to the denominator of 12. For oatmeal, 3/4=9/12. For sugar, 2/3=8/12. For milk, 4/6=8/12. We see that sugar and milk have the same measurement, and that oatmeal has the most. So oatmeal has the largest amount.
Tell me if this helps!!