This will be a really short post because I only had fifteen minutes to spare in the Michigan Room of the Three Rivers, Michigan library. While I couldn't accomplish much in an unfamiliar repository in so short a time I was able to find a few things in city directories.
If you have relatives who lived in Three Rivers you may be interested in searching for them in the library's collection of city directories. A member of the library staff told me she believed the library possessed a full run of city directories, but I cannot confirm that. The earliest directory I found was for 1871. A large gap followed before the next city directory in 1905. 1908-9 was the next one. From 1922 (the next one) up to the 1950s Three Rivers city directories were published about every two years. In my haste, I forgot to take note of the city directories later than this. After all, I was trying to make the most of my visit.
One nice thing about the Three Rivers city directories I examined (those in the 1930s and 1940s) was that all household members were listed. In addition, household members serving in the military during WWII were noted. I only wish I had had more time to examine the directories more closely. I took non-flash photographs of the pages relevant to me, along with the title page, of course and moved on.
On the other side of the room I also found a single St. Joseph county directory for 1880 next to a binder with an every name index compiled by volunteers. While I knew in what township I would find my family, for those with kin scattered throughout the county this index will prove valuable in expediting the search.
One last resource I took a very brief look at, because I had to leave, was an index card file with information on local individuals. The card on the one person I looked up had death information and not much more. Those with ancestors who lived in the area for a long time may find more in the many drawers in this collection.
The Michigan room with its large table for research definitely has local resources for those with Three Rivers roots. For general information about the resources available here I refer you to the Michigan Room page on the library website.