Commentary for Kilayim 8:3

The person driving [the two different animals yoked together] receives the forty [lashes]. And the person sitting in the wagon receives the forty [lashes]. But Rabbi Meir exempts [the latter]. This section explains who exactly is liable for transgressing the laws of kilayim, or cross-yoking, of beasts/wild animals. According to the first opinion, there are two essential prohibitions which one can transgress driving the kilayim animals and sitting in a wagon being driven by the two animals. Although the one sitting in the wagon is not the one leading or actually causing the plowing to be done, he is still liable. The Yerushalmi explains that by sitting in the wagon he balances the weight and thereby contributes to the plowing being done. Both of these people have transgressed the biblical commandment and hence they are liable for lashes, the biblical penalty applicable to one who has transgressed a negative commandment. When the mishnah says that one “receives the forty [lashes]” it is expressing that one has transgressed a biblical commandment. Rabbi Meir holds that only the one who actually leads the wagon is liable. Evidently, he holds that the one who sits in the wagon does not contribute to the plowing and is therefore not liable.

[The tying of] a third [animal different from the two already harnessed to a wagon] to the straps [of those animals] is prohibited. In this case two animals of the same species have already been yoked together when someone yokes a third animal, different from the first two. This is prohibited. Assumedly, one might have thought that this was not prohibited because the Torah only mentions one who yokes two different animals together. The mishnah comes to teach us that even three (two of which are the same) are prohibited.