Kilayim, Chapter 3, Mishnah 4. The Mishnah deals with planting rows of three kinds of vegetables alongside one another, and with the question of when they appear as separate fields and when as a forbidden mixture of kilayim.
Two rows of each kind:
"Notei'a adam shtei shurot shel kishu'in, shtei shurot shel delu'in, shtei shurot shel pol haMitzri - mutar" - a person may plant two rows of cucumbers, two rows of gourds, and two rows of Egyptian beans, and it is permitted. The Rambam adds that a furrow separating between them is of course required. Each of these groups of vegetables, since it consists of two rows, looks like a field of its own; therefore, as long as there is a furrow between them, it is permitted and they do not appear as kilayim.
One row of each kind:
"Shurah shel kishu'in, shurah shel delu'in v'shurah shel pol haMitzri - asur" - a single row of cucumbers, a row of gourds, and a row of Egyptian beans is forbidden. Here it looks like a mixture. The leaves of these three kinds tend to spread out and become intertwined with one another, and since there is only one row of each kind, they appear as though they were sown as kilayim.
The intermediate case - the dispute between Rabbi Eliezer and the Sages:
A row of cucumbers, then a row of gourds, then a row of Egyptian beans, and after that another row of cucumbers - so that the cucumbers are on both sides and the two other kinds are in the middle.
Rabbi Eliezer permits: the two outer rows, both of cucumbers, define the place as a field of cucumbers. Within this cucumber field a row or two of another kind of vegetable was sown, and if there is a furrow between them, that is enough, as we learned in the previous Mishnah. They are not regarded as individual rows but as a field of cucumbers.
And the Sages forbid: since the two rows of cucumbers are not next to one another, as was the case in the first clause of the Mishnah, where each kind had two rows so that it could look like a full field of cucumbers, of gourds and of Egyptian beans - this is not viewed as a field of cucumbers but as individual rows. Consequently the case falls under the second scenario of the Mishnah, in which individual rows of these vegetables appear as though sown in a jumble, mixed together, and it is therefore forbidden.
In summary: two rows of each kind, separated by a furrow, are permitted, since each kind looks like a field of its own; one row of each kind is forbidden, since the leaves become intermingled and it looks like kilayim. In the intermediate case, where the rows of cucumbers are at the two ends, Rabbi Eliezer permits, viewing it as a field of cucumbers, while the Sages forbid, viewing it as individual rows sown in a jumble.