Unlike Shinmei-Gū Jōge-niza, Sansha Jinja is one of the 448 shrines for which the Nagano-ken Association of Shintō Shrines offers some information. Unfortunately it is very limited, and its "history" section, just 65 characters long, is more descriptive than historical. It begins by telling us that the main deity is Amaterasu Ōkami and there is an image of some kind of her on the roof (at least I am guessing this is what it tells us: instead of a kanji there is a black box in the original and this is very confusing. For those of you who read Japanese the original is here). It goes on to tell us that as the shrine is situated in the midst of dry field farming land the typical offerings at the festival held in honour of Toyōuke-Ōkami, a grain-related kami, seeking an abundant harvest are are lettuce and daikon.
(Note: numbers in parentheses after kami names
Tateminakata-no-mikoto (111) 建御名方命
From Merged Shrines
Iwadare Jinja 岩垂神社
Annual Festival: September 27 and 28
Equally unfortunate is that when I went to visit the shrine a large part of its grounds was roped off to allow construction work to take place so I was not able to explore as much as I would have liked. I suspect though that I didn't miss too much. It is about 5km away from Hirooka Station, and is probably of little interest to anyone except the most dyed-in-the-wool jinja jaunter.