Translation:Arukh ha-Shulchan/Orach Chaim/550
All are required to fast on these four days, and it is prohibited to 'break through the boundary' [by ignoring them]. Even one who is suffers a malady of the eyes should not take these fasts lightly. In fact, all who suffer a malady, as long as they are not the type exacerbated by fasting, like a malady of the head or heart, should fast. A pregnant of nursing woman should not fast on any of these days aside from Tisha b'Av. Healthy [pregnant or nursing] women do have custom to be stringent with themselves and fast on these three days as well. Even though it has been established that these women are not stringent in respect to fasts decreed because of lack of rain, as I will write in chapter 575, nevertheless in regard to fasts established for the entire Jewish people they are permitted to be stringent with themselves. If, however, the fasting puts them in much discomfort it is prohibited for them to be stringent with themselves even if it poses no danger to to her or her baby (see the Magen Avraham in comment 2). Similarly, one who is bodily ill should not be strungent upon himself to fast, except on Tisha b'av.
On these three fast days [aside from Tisha b'Av] it is permitted to wash, anoint oneself, where [leather] shoes, and have marital relations with one's wife. One does not have to desist from these while it is yet day [IT IS ALREADY PERMITTED, WHAT DOES THIS CLAUSE MEAN?] since nowadays the only fast that has the legal status of being a communal fast day is Tisha b'Av.
All of these fasts - should they fall on Shabbos they are pushed of until after Shabbos. As to the question of why they are not scheduled for Friday instead, like the fast of Esther [before Purim] - the reason is that negative events are not advanced to occur sooner [unlike the fast of Esther, which did not end negatively]. The only exception to this is the fast of the tenth of Teves, which, according to our calindrical system can never fall out on Shabbos, only on Friday. This fast is held on Friday and completed, with the fast day Torah reading of "va'Yichal" in the morning and at Mincha time. Now even though we see historically in the Talmud (Taanis, 26.1) that the Men of the Ma'amad [The Mishnah (on 26.1) teaches that all of Jewish People were divided into twenty-four Ma'amados (groups) by the early prophets. Each Ma'amad includes Kohanim, Leviyim and Yisraelim. The Ma'amados were responsible to see that the sacrifides in the Beis ha'Mikdash were sacrificed properly for one week at a time. Thus each Ma'amad was on duty approximately twice a year] did not read this portion on Friday because of the honor due to the Shabbos, that was because they did not even fast on Friday, as is explained there. This, however, is not comparable to a official communal fast [which is more serious in this regard. Furthermore, the situation with the Ma'amad occurred every week, whereas a similar situation with a communal fast only occurs one in many years (see the Magen Avraham in comment 6 - in my humble understanding it appears to be as I have written here.)
Though as a matter of law there is no prohibition against washing with hot water on these other three fast days, nevertheless the custom is to consider it prohibited, with the exception of Friday, when one can wash in honor of the Shabbos (Bach). This is, however, no custom to prohibit washing with cold water, and one may do so. One may also sit in steam room [bathhouse] when it is not pleasure that is intended but rather the removal of the discomfort of sweating.
Our teacher the Beis Yosef writes in section 4 [of his work] that on Shabbos preceding a fast day the leader of the prayers in the synagogue announces the coming fast, except for Tisha b'Av, Yom Kippur and the fast of Esther. The Mnemonic for this is [a phrase borrowed from Mishlei 16.26]: אכ"ף עליו פיהו, where the letter 'alef' stands for Av, the letter 'Kaf' stands for Kippur, and the letter 'Pey' stands for Purim. The reason for this is that these three fast days do not depend on the will of the community, whereas by other fasts, such as the seventeenth of Tammuz, the fast of Gedalia and the tenth of Teves, the Talmud states "If the community wishes to fast they do, and if not, not." Therefore we announce these fasts to proclaim to all that it is the intention of the community to fast. Since nowadays all these fasts are not dependent on the will of the community, and we have accepted them all as obligations, our teacher the Rema writes that "the custom in the Ashkenazic communities is that none of the fasts are officially announced in the synagogue", since now none of these are dependent on the will of the community (Gra).