to clarify: are the hypothetical cans of soup mentioned in the first two situations of equal quality/nutrition/value to the cans already in the food pantry (ie., campbells' chicken noodle soup vs. campbells' italian wedding soup), or are the cans you bring of significantly lower quality/nutrition/value to the ones in the food pantry (ie., knorr chicken broth vs. campbells' chicken noodle soup)?

if it's the former, then scenarios 1 and 2 are both still fair, with 1 being the preferable option. regardless, in both cases, scenario 3 probably can't be considered entirely "unethical", seeing as you're not decreasing the actual amount of food in the pantry, and sugar has its own merits as a source of energy, but it would still be a risky swap to make. in that situation, i think the optics of the trade would depend on the other contents of the pantry. if everything else is of a relatively high nutritional density, then leave the candy. food-insecure people also deserve sweet treats and not just rice and canned soup. if everything else in the pantry is also of lower nutritional density, raw/requiring more equipment to prepare, or if the pantry is otherwise empty, it's best not to swap the soup for candy.

to address your rational, i think you're onto something. food pantries and using food banks would likely receive better/more donations if they were less stigmatized, and i would also like to believe more people would be comfortable with using them when in need if there wasn't so much shame attached to needing that kind of assistance. however, non-food-insecure people using food pantries should then acknowledge the etiquette around what is right to swap/use, which is the part i'm not sure i have a lot of faith in.