Who Knows Three? Four? The Real Magic Number of the Seder

What differentiates this night from all [other] nights? On all [other] nights we eat chamets and matsa; this night, only matsa? On all [other] nights we eat other vegetables; tonight (only) marror. On all [other] nights, we don't dip [our food], even one time; tonight [we dip it] twice. On [all] other nights, we eat either sitting or reclining; tonight we all recline.

(3) [Then] they set [food] before him. He dips the lettuce before he reaches the course following the [unleavened] bread. [Then] they set before him unleavened bread, lettuce, and a mixture of apples, nuts, and wine, and two dishes, although the mixture of apples, nuts, and wine is not compulsory. Rabbi Eliezer bar Tzadok says: It is compulsory. And in the Temple they used to set before him the Passover-offering.

(4) They pour a second cup [of wine] for him. And here the son questions his father. And if the son has insufficient understanding [to question], his father teaches him [to ask]: Why is this night different from all [other] nights? On all [other] nights, we eat leavened and unleavened bread, [but] on this night, [we eat] only unleavened bread. On all [other] nights, we eat all kinds of vegetables, [but] on this night, [we eat only] bitter herbs. On all [other] nights, we eat meat roasted, stewed or boiled, [but] on this night, [we eat] only roasted [meat]. On all [other] nights, we dip [vegetables] once, [but] on this night, we dip [vegetables] twice.

(8) They shall eat the flesh that same night; they shall eat it roasted over the fire, with unleavened bread and with bitter herbs.

Rabban Gamliel was accustomed to say, Anyone who has not said these three things on Pesach has not fulfilled his obligation, and these are them: the Pesach sacrifice, matsa and marror.