At runtime it depends. If we're using arbitrary outside integers which might be three, we're obliged to check yes, nothing is for free. But perhaps we're mostly or entirely working with numbers we know a priori are never three.

NonZero<T> has a "constructor" named new() which returns Option<NonZero<T>> so that None means nope this value isn't allowed because it's zero. But unwrapping or expecting an Option is constant, so NonZeroI8::new(9).expect("Nine is not zero") will compile and produce a constant that the type system knows isn't zero.

Three in particular does seem like a weird choice, I want Balanced<signed integer> types such as BalancedI8 which is the 8-bit integers including zero, -100 and +100 but crucially not including -128 which is annoying but often not needed. A more general system is envisioned in "Pattern Types". How much more general? Well, I think proponents who want lots of generality need to help deliver that.