The standard 18" alloys on the A3 always came with 225/40-18. Anything different from that you should inform your insurance company as the tyre size has moved away from stock for the wheel.
You dont need to tell the insurance company, or at least mine have never questioned the tyre profiles.
They only care about value of wheels or if they are more likley to be stolen.
They dont care about handling or they would ask the brand and charge you more for budget than premium tyres.