When is it a good time to change tyres - every three years, or after clocking 30,000km?
The best gauge would be the wear indicator, a raised nub found between the main grooves of your tyres.
Once the tyre surface is level with the indicator, it is time for a change.
It does not matter when this happens or after how many kilometres of driving.
But if your car has clocked 30,000km and the tyres have not reached the wear indicator, you should still consider changing them because the rubber may have hardened and grip may be compromised.
Do not scrimp on tyres because you never know when you will need maximum performance.