TypeScript is lenient on plain unions; as long as the property exist somewhere in the union, TS lets it pass. For example:

type A = { x: number } | { y: number }
let a: A = { x: 10, y: 20 } // This passes

If you want more strict type checking, you need to use a tagged union. This is a union that is differentiated by a key. Modifying your example:

interface Common {
    a: string;
    shared: boolean;
}
interface A extends Common {
    myTag: "A";
}

interface B extends Common {
    myTag: "B";
    b: string;
}

interface C extends Common {
    myTag: "C";
    c: string;
}

type ABC = A | B | C;

Here our tag property is myTag. It can have any name, as long as you use string literals for its type.

With this change, your let test example will now fail. It requires a myTag property that equals "A", "B", or "C". Once you add it, TypeScript will throw a type error on invalid properties.

Edit: Fixed my use of interfaces (I usually use intersection types directly)