The Three Stooges are in the medevial era in "Squareheads of the Round Table", released in March 1948. Moe, Larry, & Shemp have to help a lovestruck blacksmith win his true love, who has been promised to the evil Black Prince.
Edit, 5/11/18: Had to change the video. The new copy is a little longer, and as a result, the music at the start has slowed down to avoid copyright issues.
I believe that was a form of a mandolin that Moe is playing opposite Larry's violin and Shemp's squeezebox (an early accordion).