Assuming the tyres were all new or close to new when you purchased the vehicle, the fact that three of the tyres are significantly worn but one is not would suggest that the wheels are not correctly aligned, although incorrect tyre pressures could also cause premature wear. We would suggest having the tyres replaced and a full four-wheel alignment carried out at the same time. You can go back to the dealer and ask for a contribution, but they may consider this outside the normal checks they would perform when selling a used vehicle.