2 spools are turned on a dual spool, 3 spools on a triple spool - the HP directly by the starter and the other/s due to airflow induction.
The easier starting aspect is to do with the stability of the airflow through the engine to the combustors. I triple spool engine will act like a dog if start attempts take place too far below max motoring. You'd expect to see 2/3% N1 ( not too accurate at motoring speeds ), between 5-11% N2 ( the higher the better generally ) and low 30 something %'s from a good starter on a triple spool. Not textbook but it works.
