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Origin of the word
In Japanese Sansuke means three kinds of services, kamataki (stoking of the boiler), yukagen wo miru (checking the temperature of the bath water), and bandai(fee collection). It was an occasional duty for the Sansuke to provide the service of washing off dirt or brushing for customers. For this reason, the image Sansuke = the service to the visitor in the bathhouse was generalized emphasizing this role.
When smallpox was occurring around in Nara era, the Kohmyo Kougou (光明皇后 Empress Kohmyo ?), the Shohmu Tennoh (聖武天皇 Emperor Shomu?) 's empress, build the bathhouse and devoid to the treatment of the patients by herself. Three chamberlains helped the Empress during that time. They called them Sansuke(三典), and this name became the origin of Sansuke (三助) thereafter.  During the Edo era, a Genan (下男 manservant?), or Komono (小者 humble servant?) was wildly described as Sansuke too.
Sansuke in premodern times
Until the early Edo era, these services were provided by a Yuna girl, but gradually it changed to sexual services. At that time, Okami (the government) inhibited these services.
With the prohibition of the Yuna girl, the role was replaced by a Sansuke boy. Sansuke was a supreme class of the male servants who serve a master of the zeniyu. To become a sansuke, there were several precursor roles: the collector of firewood, the boiler man, and Yuban, the checker of the bath temperature and so on. . Yuban had the important work of checking the congestion degree of the bath or bathroom in addition to above work and it was possible to become a Sansuke by gaining a lot of experience.
The service of washing off the dirt and brushing done by a Sansuke is Nagashi. When announced by Bandai that there are customers who want his service, a Sansuke laves hot water into Oke and calls the customers in. In case there are many customers, the Sensuke must take care of the waiting list and must wash the customers swiftly. The Sansuke offers service for both men and women; he may be required to work in crowds of naked woman, whereby he must adopt the mental training to prevent himself from having an erection in front of women.  It is said that women were not ashamed of his eyes.
When the washing service was done and received a tag from customer, his work was accomplished. His percentage of pay was based according to the number on the tag. The Sansuke belonged to a higher economic bracket in Sento and were considered to be gentlemen.
Kodakara no yu and Sansuke
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In various parts of Japan, there existed Kodakara no Yu (the magical spa where the infertile women are able to conceive). Nowadays it is thought the magic was that the Sansuke engaged in sex with a female customer and made her pregnant. If the cause of the woman's inability to conceive was due to the husband's infertility, then the Sansuke would allow for a successful pregnancy to occur. (Not that the Sansuke or the spa had any magical powers, the Sansuke fathered the child).
During the Edo era, people had different priorities. For women, it was of paramount importance to leave the household with a son and heir; thus, they had to become pregnant by some way. A daughter-in-law who had not yet produced a son would be given directions to go Kodakara no yu impliedly by mother-in-law and father.
Sansuke during early-modern times
Aka-suri or massage was called a Nagashi, and it reached heights of prosperity in the middle of the Showa era; during this time it was considered to be a luxury. But according to diffusion of boiler or bath in general household, the need for Sansuke was diminished and the service of Nagashi ebbed away. 
Sansuke of today
According to Shinobu Machida (町田忍), the historian of zeniyu, there is a[clarification needed] only sansuke at Saitoh yu (斉藤湯) in Tokyo Japan. His name is Mr. Shusei Tachibana (橘秀雪). He gives service for both men and women now. Of course they do not engage in side work (that is to say, the pre-modern fertility treatment), they give Akasuri or massage treatment only. At work, Sansuke put on Sarumata (Japanese briefs); in former days Sansuke put Fundoshi (breechcloth), his style was thought naughty by women. 
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