Most manufacturers cartridges have a deeper body than Rega cartridges, these require the use of the three point spacer to allow clearance of the rear of the cartridge from the vinyl. In just about all cases you will need two of these if installing an aftermarket cartridge. If you measure a Rega cartridge height (from the stylus tip to the top), it's about 14mm. Most cartridges measure 18mm or so.
These are easy to install and we install them often when putting cartridges such as the Ortofons, Hanas, Sumiko's, Clearaudio's on Rega turntables. If you do install it yourself make sure you are using a Pozidrive screwdriver if you have Pozidrive screws or a standard screwdriver if you have standard screws.
Note that with two of these on, and the standard tonearm weight (P1,P2,P3,P6), you will not be able to close your lid. To remedy, you need to install a Rega Tungsten weight which is lower in diameter. The Tungsten weight will make your table sound a little better.