Why do some lorries have three axles?

Weight distribution. In the UK and EU, the 4x2 configuration is the "norm", and there can be up to 40,000 lbs. on the drive axle. This is very strenuous on the asphalt, and some trucks have an additional lifting tag or pusher axle to better distribute the weight, thus reducing the strain placed on the roadway. Vehicles intended for heavy haulage have a non lifting axle, and are in a straightforward 6x4 configuration. Again, this is for weight distribution.