Our Mishnah opens with a description of the technical details of the terumas halishkah, the appropriation from the Temple chamber, which was carried out three times a year: the removal of funds from the room in which all the half-shekels had been collected, in order to purchase the public offerings with them. The Mishnah teaches that this entire procedure was performed in sets of three.
The pattern of threes:
The appropriation was made three times a year.
Each time, the funds were taken in three baskets.
Each basket held three se'ah.
On the three baskets were written aleph, beis, gimmel, so that it would be clear which had been filled first, which second and which third. The money used for the purchases was taken first from the first basket, and when it ran out they moved on to the second, and so on.
Rabbi Yishmael adds an interesting detail: the baskets were not marked with Hebrew letters at all, but with the Greek alphabet - alpha, beta, gamma, its first three letters.
The clothing of the one who performs the appropriation:
The Mishnah continues and teaches how the man engaged in this task would enter, for he must be above all suspicion, so that no one should say that he hid some of the coins in his clothing while he was there. Therefore he would not enter:
In a garment with a hem or a cuff - lest he hide a few shekels in it.
In a shoe or a sandal.
In tefillin, or in anything else that has a receptacle.
In an amulet.
Anything in which a coin could be concealed he would not wear. And the reason: perhaps he will become poor, his fortune will turn, and people will say that he became poor as a punishment for misappropriating the consecrated funds; or perhaps he will become wealthy, and people will say that he grew rich by stealing from the appropriation. Either way, suspicion is liable to arise.
One might argue that such suspicions are not reasonable, but the Mishnah teaches that a person is obligated to place himself above human suspicion. Just as he is careful that there be no grievance or suspicion against him from Above - and although with the Holy One, blessed is He, it is not really a matter of "suspicion," but rather that a person seeks to be on good terms with the Holy One, blessed is He, as it were - so too must he seek to be on good terms with his fellow human beings.
Two sources for this from Scripture:
"Vih'yisem neki'im meiHashem umiYisrael" - "And you shall be clean before Hashem and before Israel," a verse stated regarding the tribes of Reuven, Gad and half of the tribe of Menasheh, teaching that one must be clear of suspicion in the eyes of people as well.
"Umtza chein v'seichel tov b'einei Elokim v'adam" - "And you shall find favor and good understanding in the eyes of God and man," in the language of the verse in Mishlei.
In summary: In this Mishnah we learned the technical details of the appropriation from the Temple chamber - the pattern of threes (three times a year, three baskets, three se'ah), the marking of the baskets with aleph, beis, gimmel to establish the order in which they were used, and the statement of Rabbi Yishmael that the marking was done with alpha, beta, gamma. We also learned of the care taken regarding the clothing of the one performing the appropriation, that it contain no receptacle of any kind, and the great principle that emerges from this: a person's obligation to emerge clean of suspicion not only before Heaven but also in the eyes of his fellow human beings.