Sanshamairi is to visit three Shinto shrines. In some regions sanshamairi is a custom which has taken root in general.
A custom called 'Sanshamairi' at various places in western Japan
Sanshamairi which is a custom at various places in western Japan means visits to three Shinto shrines for hatsumode (new year's visit to a shrine) on New Year's Holidays.
The people who live at the places where this custom has been established do sanshamairi as a matter of course regardless of particular special (religious) piety, considering 'hatsumode = sanshamairi.'
It is a custom which has firmly taken root especially in parts of Kyushu and Chugoku regions centering around Fukuoka Prefecture.
Visiting multiple shrines on New Year's Holidays sounds a little peculiar to the people who do no have a custom of sanshamairi and some of them get it negatively saying 'It may reduce appreciation,' and 'It may be not so good.'
In some areas we hear rules such as 'You must visit three shrines within one day.' or 'You must make the visits on the first three days in January.', but there are no fixed rules.
While some question which shrines are meant by 'sansha' (three shrines), in some areas a fixture is clearly determined and, in other areas it is totally unclear and any three shrines you like can be visited. However, if there is a famous large shrine, it is natural to include that shrine.
We hear no fixed rule concerning the order of the visits to shrines.
The origin of sanshamairi
There are various theories concerning the origin of sanshamairi, but it is not clear.
One theory says that originally the Chotei (Imperial Court) conducted hohei (offering a wand with hemp and paper streamers to a Shinto god) at the three shrines, namely Ise Jingu Shrine, Iwashimizu Hachiman-gu Shrine, and Kamo-jinja Shrine, which has spread to the general public as a custom to visit three shrines.
Another says that it is meant to salute to the three shrines, namely ujigami (a guardian god or spirit of a particular place in the Shinto religion) who is an ancestral guardian god, ubusunagami (guardian deity of your birthplace), who is the guardian god of your birthplace and the guardian god of your entire life, and chinju (local Shinto deity) who is the guardian god of the place where you live.
Regions where the sanshamairi custom exist.
Sanshamairi in Wakayama Prefecture
They visit three shrines, namely Hinokuma-jingu Shrine/Kunikakasu-jingu Shrine, Kamayama-jinja Shrine, and Itakiso-jinja Shrine located in Wakayama City, Wakayama Prefecture. Many people have been visiting the three shrines through the ages, and Kishigawa Line of WAKAYAMA ELECTRIC RAILWAY Co., Ltd. is available connecting these shrines. Hinokuma-jingu Shrine, Kunikakasu-jingu Shrine, Kamayama-jinja Shrine were formerly designated as Kanpei-taisha (large-scale state shrines), and Itakiso-jinja Shrine a Kanpei Chusha shrine (middle-scale state shrines).
The nearest stations
Hinokuma-jingu Shrine/Kunikakasu-jingu Shrine
Hinokuma Station on Kishigawa Line of WAKAYAMA ELECTRIC RAILWAY Co., Ltd.
Kamayama Station on Kishigawa Line of WAKAYAMA ELECTRIC RAILWAY Co., Ltd.
Itakiso-jinja Station on Kishigawa Line of WAKAYAMA ELECTRIC RAILWAY Co., Ltd.