Shevi'is, Chapter 1, Mishnah 5. In the previous mishnah we learned the principle of a field of trees - an orchard or grove - which may be plowed in the sixth year until Atzeres (Shavuos). And if it does not contain three trees, or the three trees do not produce fruit in the required quantity, it is considered a grain field, which the Sages call a 'white field', and it may be plowed only until Pesach.
A further qualification in the law of the three trees:
Our mishnah teaches: "Sheloshah ilanos shel sheloshah anashim - harei eilu mitztarfin, ve'choresh es kol beisa bishvilan" - that is, even if the three trees belong to three different owners, they combine with one another, the area is considered a field of trees, and the entire beisa (the beis se'ah surrounding them) may be plowed on their account until Shavuos.
And how much space must there be between them?
The mishnah now discusses the distance required between the trees: "Amar Rabban Gamliel: kedei sheyehei habakar over b'chelav" - the amount of space between the trees must be enough for an ox to pass through with its implements, meaning the tools used for plowing, and that measure is four amos.
And if there are not four amos between them, we assume that the trees stand so close to one another that they are destined to be uprooted, and therefore they do not count as one of the three trees on whose account it is permitted to plow until Shavuos.
In summary: In this mishnah we learned two laws in the definition of a field of trees: three trees combine even when they belong to three different owners, and the entire beisa may be plowed on their account; and the space between them must be enough for an ox to pass through with its implements - four amos - for if not, they are destined to be uprooted and do not join the count.