Well "stay" is a pretty hard and unnatural one for a dog, they're supposed to follow the other pack members around! XD I taught mine to "wait" (as in wait for a bit at the side of the road til we're good to walk, or wait while I'm putting the bowls down and you're not allowed to eat til I say when) but for my dog didn't work "stay" too hard as I was training him as a tracking dog so it didn't seem like a necessary thing at the time.

He'll get there with the other two, though if he's anything like my dog stay will be a lot harder than the other two XD

Have fun with the training :D