Your 3" GP has the Lett manufactured Ruger COMPACT grip. These were factory issue on the Fixed sight GP100s. The Fixed sight GP100s were 3" and 4" guns. Most 3" 357, and some 4" 357s were fixed sight. There were a very few 3" GP 357s with adjustable sights. Most 3" .38 Special GPs were adjustable sight. They also shipped with the COMPACT grip.
The COMPACT grip inserts were wood(Goncola Alves), and were the same as the SP101 inserts, with the exception of having a relief hole drilled on the back side, for the larger GP100 grip locator pin. If you look on the backside of the standard SP101 plastic insert, you will see the relief hole, and the plastic insert works in the COMPACT grip without any modification. I have original catalogs from the early days of the GP100(86-87) that shows the COMPACT grip with the plastic inserts, but I dont know if any ever shipped with the plastic inserts.
The SP101 was available with wood inserts as an extra cost option, and the wood inserts were available as an accessory from the Ruger parts department. These were drilled with the relief for the COMPACT grip, and were listed as fitting both SP and GP grips.
The newest SP101s that come with the checkered, and engraved inserts(2014), have the relief hole drilled, and will work in the COMPACT grip. The first style checkered, and engraved insert(2012), does not have the relief hole.
Many people prize the GP100 COMPACT grip, because its an excellent grip. Its just barely a full size grip, and allows better concealment for the smaller GPs. They bring anywhere from $40-$250 on the auction sights.
This is the first style checkered, and engraved insert, that isnt factory drilled. I drilled the relief, and I like the look of this insert. This is my 3" 357 GP100.