The Nesco roaster has side heat, not good for sauteing. The Hamilton Beach and Rival have bottom heat, better for that part, not as good for the casserole/holding part. You will have to adapt your cooking to the particular roaster.

Also CAUTION; 2 roasters on 1 15 amp circuit will blow the circuit and even if you do just 75 pounds, you are looking at 6 roasters! Check you venue in advance; most cannot handle this electrical load.

You can't saute that many frozen beans in a pan that size; it won't stay hot enough. Visualize the size pan you would need to do just one 5 pound bag on the stove, and you will get the picture.

This recipe for frozen baked green beans works, but see how 1 pound of green beans takes a 1 1/2 quart casserole? And it gives a well cooked bean, not the lightly cooked bean I think you are aiming for.

16 ounces frozen French-style green beans, thawed
1 teaspoon bouillon granules (1 cube)
1 cup hot water
1/2 teaspoon garlic salt
Pinch pepper
1 tablespoon butter

Put the beans in an ungreased 1 1/2-quart baking dish. Dissolve the bouillon in the hot water then add the garlic salt and pepper. Stir in the butter and pour this over the green beans; stir well. Cover the baking dish with foil and bake at 350º for 45 minutes.

So you have some serious thinking to do about this idea.

You might consider to roasted vegetables made ahead and served room temp, Italian syle, for this large a group. They do make a prettier plate, as well!

Here is the official Hamilton Beach Roaster green bean recipe, less added salt:
10 packages. (10 oz. each) frozen green beans= about 7 pounds
8 quarts hot water (reduce if adding tomatoes)
6 smoked ham hocks (I often use smoked turkey if here is any question of non pork eaters. You can also sub 6 cups of diced tomatoes in juice plus 1 pound diced sauteed onions for a delicious vegetarian option)
1-3 teaspoons pepper
1-3 teaspoons sugar or brown sugar

Directions
Preheat Roaster Oven to 375°F.
Place all ingredients in roaster oven insert pan. Liquid should just cover beans. Cover and cook for 4 hours.
Add more hot water if necessary to keep beans covered.

Serves: 30

Just 2 roasters for beans. You can do at least 3 as you propose, just plug in and turn on at the same time days before you plan to cook to make sure they are on different circuits.

Don't add plain water, they are already minus some of their flavor. I would not use butter. When I do canned green beans, I either go country with chopped onion, maybe add ham or bacon in one roaster; or Greek with adding a can of stewed tomatoes, onions, garlic, cinnamon and sugar

Country
5 #10 cans uses about 1/4 cup of sugar, 5 cups shredded onion, 3 cloves garlic, 2 tablespoons black pepper, 1/2 pound shredded smoked turkey or cooked bacon. You saute the shredded meat and onions in oil with the pepper before adding to the beans and you add enough liquid to cover. The roaster will be full.

I use turkey for large groups because many people prefer not to ea pork.

Greek
For this, drain off the juice, but save to add back.
Each #10 can takes 2 tablespoons sugar, 2-3 cups chopped onion, 3 cloves garlic, 1 tablespoon dried oregano, 2 teaspoons ground cinnamon and a small can of stewed tomatoes.
So 5 #10 cans uses about a generous half cup of sugar, 10 cups chopped onion, 10-12 cloves garlic, 1/4 cup dried oregano, about 2-3 tablespoons ground cinnamon #10 can of stewed tomatoes. The roaster will be VERY full.
You cook the onions in olive oil before adding to the beans with everything else. Add back just enough bean juice to bring to the top of the beans.

Preheat the roasters at least 20 minutes before you start counting the cooking time, and consider 300 the first hour. Keep he lid on; like crock pots you add to the cooking time every time you open the lid.

If it were me, I would seriously consider using 3 roasters, on 3 different circuits, to allow for even heating and easy stirring.