The 3 and the 2.5 is the extra height addition to the stock vehicle height nor the spacers thickness of coarse
Because of the suspension geometric wishbone construction it is not 1 to 1 scale.
If you want to build your own spacers and you need this dimensions,jack lift the vehicle frame for say 25.4mm ,make a spring coil to coil dimension before minus after you lifted the vehicle and compare to the 1 inch dimension (front and rear springs separately)
The maximum spacer thickness you could use without changing the strut upper studs is approx 8mm.the front spacer must be higher than the rear for approx 30 percent to get the vehicle leveled.
If plastic spacers used, be aware of the plastic torque deformation.i mean, nylon spacers will need to be retorqued approx twice after vehicle suspension seated