Changing employers/jobs every three years:
Some Poskim rule that an employee, including a worker, teacher, [Sofer], or servant, is not to work for the same employer in his home on a permanent basis for more than three years. Nevertheless, if one does not have another source of sustenance to provide him food, [and has nothing to sell] then he may continue working even past three years. (However, a woman is not to work for the same employer more than three years, according to the above opinion.) [Other Poskim rule one may work for the same employer for over three years, although may not work for the same employer for six years. Other Poskim however argue on all the above and rule that there is never a prohibition involved in working for an employer for more than three years, in any situation.]
Some Poskim rule one may not work for the same employer for more than three consecutive years, unless he has no way to support himself otherwise and will be left hungry. See Q&A as to the form of employment this prohibition includes!
Does the above prohibition [according to stringent opinion] apply towards any job, or only towards a job in which one works in another’s home and lives there on a permanent basis during his years of work?
From the wording of the Poskim it is implied that the prohibition only applies when the worker lives at the house of his employee and is supported by him in terms of room and board. In such a case, they stringent opinion holds it is forbidden to work in such a way for more than three years due to it being similar to a slave. However, if one lives in his own home and eats his own food, then seemingly there is no issue according to any opinion, against working for the same employer for many consecutive years. Nonetheless, some Poskim rule that the above prohibition applies even by an employee who does not live in the home of the employer, such as the Rav of a community, as nevertheless his room and board is supported and payed for by his employer, which are the people of the city. It is for this reason that the contract of Rabbanim is either for three years or five years but not for six years.
 Admur Hilchos Sheila Usechirus Vechasima 21
 Yeish Mi Sheomer in Admur ibid; Rama C.M. 333/3; Darkei Moshe C.M. 331; Shach C.M. 333/17; Hagahos Mordechai Perek Haumnim Remez 459-460; Maharam Merothenberg Teshuvah 85; See Shabbos 127b “There was a certain man from the upper Galile who hired himself to an employee for three years”
 Rama ibid; Hagahos Hamordechai
 So is wording in Admur ibid; However in Rama ibid the wording is “Therefore it is forbidden..”
 See Q&A!
 The reason: As up until three years of work one is considered an employee, as it says in Yeshaya [16/14] “Beshalosh Shanim Kishnei Sachir Venikleh” while by an Eved Ivri the verse [Devarim 15/18] states “Ki Mishneh Secha Sachir Avdecha Sheish Shanim”, and thus if one works for more than three years he leaves the status of an employee and enters the status of a slave and the Torah stated that “Ki Li Bnei Yisrael Avadim”, that they are Hashem’s slaves and not slaves of slaves. [Admur ibid; Shach ibid; Maharam ibid; Mordechai ibid]
Other opinions: Some Poskim learn one may not work for an employer for up to three years, because three years of work is called a Sachir, and one may not be a Sachir. [Smeh 333/15; Shach ibid negates this Nussach and so rules Beir Sheva p. 112; Agudas Eizov p. 107; Admur ibid unlike Smeh] The practical ramification is whether may one work on the last day of the three years.
 Admur ibid Kuntrus Achron 4 based on Rambam Avadim 1/1; Shach ibid
 Admur ibid; Shach 333/16; Aruch Hashulchan 333/14
The reason: As in times of the Yovel, a person who had no form of sustenance could even sell himself as a slave, as the verse [Vayikra 25/39] states “When your brother becomes destitute and is sold to you”, and thus certainly he may work for more than three years. [Admur ibid; Shach ibid]
 Admur ibid in parentheses
The reason: (As even in times of the Yovel, a woman who reached puberty [growth of breasts] could not sell himself as a slave [even if destitute] [see Bava Metzia 12b; Tosafus ibid; Rambam Avadim 2], therefore she may also not work for more than three years according to the above opinion.) [Admur ibid in parentheses]
 Chasam Sofer O.C. 206; Opinion in Aruch Hashulchan 333/14
 The reason: As the verse considers an Eved Ivri to be a slave of six years, and hence any less than six years is not an Eved Ivri. [ibid]
 Tosafus Bava Metzi 10a, brought in Mordechai ibid, Beir Sheva ibid, Shach ibid, Aruch Hashulchan ibid
 Admur ibid; Rama ibid “To be in the house of the employer permanently”
 Chasam Sofer O.C. 206 and C.M. 172, brought in Pischeiy Teshuvah 333/6