I have used store-bought yeast to ferment barley. It will definitely work. You're going to get a lot of off flavors from the fermentation though. These flavors can be mitigated by using a longer and colder fermentation process. It's going to easily get all the simple sugars but the maltostrios Is going to take more time. You're probably looking at a 4 day rise at something like 50°f. Even then it won't grab it all.

But yes I did misread that thinking it was going to be three cups of barley.