Small pots are going to freeze solid under most circumstances. They can survive that. The key is to keep them from facing conditions which are colder than they can handle. I know it sounds like once they're frozen, they can't get much colder, and that's true for seeds, but it isn't true for root tissue. Give them as much protection as you can. I'd bury the pots in the ground, and put the hoop house over them as well.

Fescue is going to continue growing, right up until the coldest part of winter. Chasmanthium and Eragrostis will go dormant when the soil temperature drops sufficiently. I'd bury the Chasmanthium and Eragrostis pots first, and let the Fescue wait. Make sure the Fescue is in large enough pots to accommodate root growth through the fall and early winter. Otherwise, when you bury the pots, the roots will grow out of them and into the adjacent soil.

Make sure all the pots are well-watered in, and buried in sharp-draining soil. The Eragrostis and Chasmanthium won't need as much water as the Fescue during confinement. None of them want to sit in wet soil.

Patrick, That's good advise from Donn. The benchmark for losing a perennial grass is not freezing temps. I've used a general rule of thumb of the root-ball reaching 18 deg as a sure loss point. Varies some by variety of course, but its held fairly true for me over the years. Good luck!