If you had to feed a family of three for 2 and a half weeks on $100 what would you buy?

Pinto beans and tortillas.

I'd go to costco or bj's warehouse or sam's club and buy chicken breasts and groundbeef, maybe porkchops, a couple of heads of lettuce, liquid milk for cereal, dry milk for drinking - bj's warehouse used to accept coupons I don't know if they still do. If any one them do accept coupons then that's where I'd shop.

I would get 5 pounds of hamburger meat for $10....they sell it at the store down the road called Aldis
I would get the cheap boxs of mac and cheese which at Aldis are only 34 cents a box.
My other grocery store in town sells 5 meats for $20 for I would get $40 worth of meat.
bags of potatoes always go along way.

hamburger helper, spaghetti, chicken

You doin better than us . We usually get about 30 bucks a week .
But I would probably get a big bag of potatoes ( tons of possiblities there )
Hamburger Helper
Hamburger
Frozen veggies
Bread
Milk
Cheese ( shredded and sliced )
Tortillas / Taco seasoning
Cereal
Peanut Butter
Jelly
With just these few things , which you proabably already have a few of them you will have a bunch of different meals.
Soft Tacos
Hamburger Helper
Meat loaf / mashed potatoes / veggies
Hamburgers / french fries / veggies
You get the point , Just a few ideas . This is usually what I do .

If you're not a member of any of those clubs don't join -BJ's you didn't used to have to be a member to shop there but they'd add a percentage on to your puchase to compensate for not being a member. I'm not sure if Costco or Sam's club lets non members have day passes or not. Pathmark Supermarkets sometimes has decent large packages of meats. Foodtown Supermarkets takes coupons like Pathmark does and maybe a warehouse supermarket does, But At Foodtown you build points for certain purchases in large amounts (buy this product earn 100 extra points or 50 extra points or 200 e p) BUT YOU ALSO build points and can buy Foodtown gift cards with those points and use them that day of purchasing the gift card to help bring down cost of entire purchase. ALSO THAT DAY if you buy yourself from your points earned, a Foodtown gift card and use it that day you STILL earn points on products you buy to build up points again for anotherday.

Also at Foodtown (and no I don't work for them but the points and coupons are great great help) you can use points to bring down the cost of different specific products.

Things that can go into multiple dishes or is cheap for alot:
Potatoes
Pasta
Bread
Cheese
Oatmeal
Peanut Butter
Pancake mix

I would go to hillbillyhousewife.com. They have GREAT suggestions. I once had to feed my family of 3 on $80 for 2 weeks. No problem with her ideas. My third was a year old, so almost didn't count.