To find an answer to this question, one place to look is in Chapter 8 of Guns, Germs, and Steel. In that chapter, Diamond discusses why agriculture never arose independently, or arose but only relatively late, in some areas of the world. As part of this discussion, he looks at what causes plants to be (or not to be) domesticated.
Some of the things that cause this are very obvious. As Diamond says on p. 133, plants can be rejected for domestication for obvious reasons like the fact that “they are woody, they produce no edible fruit, and their leaves and roots are also inedible.” In theory, then, you could submit this as your answer because it does give three reasons why people might not domesticate a given plant. However, if I were assigning this question, I would want you to look for a slightly less obvious set of answers.
In this chapter, Diamond discusses why the plants of the Fertile Crescent were so good for domestication. Let us look at what he says in order to find three factors that determine whether a plant will be domesticated. First, the size of a plant’s seed matters. People want to domesticate plants that will provide a lot of food. The bigger the seed (in a grain plant), the more food it provides. Second, he says that the way a plant grows matters. If a plant grows in “in large stands whose value must have been obvious to hunter-gatherers (136)” it is much more likely that people will notice it and try to domesticate it than it would be if very few specimens of the plant grew in widely scattered locations. Finally, Diamond says that annual plants are better for domestication than perennials. This is because these plants do not put their energy into growing anything like thick stalks that can survive from year to year. Instead, the bulk of their energy goes into producing things that are edible, making them much more useful to people.
These are some possible answers to this question. Other possible answers can be found in Chapter 7, for example, on pp. 123-4.