הלכה אויף דברים 24:21

The poor have no right to take peret and olélot until the owner of the vineyard begins to harvest his grapes, as it is said, (Deut. 24:21) When [and only when] you pick the grapes of your vineyard, do not go over it again [te'olél]. And how much does he need to harvest until they [the poor] have the right [to start picking]? Three clusters, for this is enough to make a quarter of a kav.

As for two vines, and so also with other trees, two which can be distinguished one from the other may be shikhecha, but three may not count as shikhecha, as it said, (Lev. 19:10, 23:22) You shall leave them for the poor and the stranger. Only if there are two, one for the poor and one for the stranger.¹⁰⁵See Tosefta Péah 3:5.

To leave for the poor, the gleanings of the vineyard (Lev. 19:10; Deut. 24:21).