If you hit a player in stride after taking more than 3 strides normally you're called for charging. You can however take as many strides as you want and glide into a player, which is what happens most of the time because players are constantly moving. So if you hit a player while striding into them it cannot be more than 3 strides unless you stop skating and glide into them.
It is a very grey area as far as rules go, in fact the 3 strides rule is not even written into the NHL rules because some player can lay violent hits with intent to injure someone without 3 strides, it is more the unwritten law of hockey.