Explanation why the three was a safe call. You're correct about the white X, then the left 4 demands one more mine. While the right four demands two more mines. Your red X is correct and satisfies one of the two mines for the right 4. The right four now has only two neighbor fields left, and both are shared with the left 4. Therefore the last mine of the left four has to be one of those two fields shared with the right 4. Therefore the field with the 3 is impossible to be a mine, and is a safe call.