The Mishna (Demay 1:1) gives a list of different types of produce exempt from demay. In Ma'asrot 5:8 and in Shevi'it 9:1, there are two more distinct lists of things that are exempt from ma'aser and which can be taken from anyone in the seventh year.
The reason they are exempt (according to the Bartenura in all three places) is because they are assumed to be ownerless.
These three lists are entirely different. What is the relation between them?
The most literal reading is that the first list is exempt from demay, the second and the third from both ma'aser and from not buying in the seventh year. This would be a good reason to distinguish the first case (not the second and third), but if so, what is the reason to distinguish between them?
It could also be that they're all exempt from all of these cases (demay, ma'aser, shevi'it), but if so, why are they given in three different lists? Elsewhere the Mishna repeats the exact same lists in different places, but here there are three lists apparently for the same reason.
The Rambam quotes the three lists separately (Terumot 2:12, Ma'aser 13:1, Shemitta Veyovel 8:12), after each one adding וכיוצא בהן, but with some differences in presentation which might or might not be significant. In Shemitta Veyovel 8:12-13 he brings the list from Shevi'it alongside the list from Ma'asrot, but I don't understand what, if any, contrast he is making.