N1, N2 and N3 are the spools....... LP, IP and HP stages
If you look at the other post above
3 sections mean each section works closer to its optimum
N1 100% = 3900rpm
N2 100% = 7000rpm
N3 100% = 10611rpm
The RPM's are to do with speeds of the stages, the optimum speed where a blade is most efficent is near the speed of sound tip wise, so if you have a big blade such as on a RB211 it will be running near the speed of sound and if it just had one compressor, the smaller blades rearwards at the compressor will not be running as near efficent, having 3 separate stages the RPM's can vary and allow each spool to run more efficently..... in an ideal world you would run every stage independant, but the weight gain would negate this.....
Incidentally on the RB199, the 2nd spool If memory serves me correctly runs in the opposite direction to reduce the forces of having all three compressors rotating in the same direction.