In my experience you can only use one per purchase (or per container).
You could stack a store coupon, with a manufacturer's coupon and then with a check (as long as it doesn't say on the check "use like a coupon").
So for instance...you could go into Target, they occasionally have Enfamil Target coupons on their website / cartwheel / mobile coupons (store coupon), use a Enfamil coupon from the newspaper (manufacturer coupon), and then one of your rebate checks
Before I knew what I was doing with couponing when DS1 was an infant, I had one cashier stack them all when I asked her if it was OK, but I came to find out that is not allowed.
FYI -- In that case, you are still still stack with a store coupon if you can find one @GoodStuff05 (treat your rebate check as a manufacturer coupon). Couponing has gotten so much more complicated/difficult these past few years.
Re: Enfamil $5 retailer coupon/checks