i don't see why it can't be done with a 3X signal. less interrupts gives the processor more time to do other work rather than service an interrupt.

in fact, the less pulses there are, the more accurate you can get. compare 3 notches on a wheel vs 6, the timer will get reset twice as often with the 6 pulse. at 6000RPM, with 6 pulses per rev, that is a frequency of 600Hz. assuming the timer ran at 65,536Hz(that way it will fill up a 16 bit counter once per second), then we're looking at a period of 1.66666mS and a timer value of 109.22(which will get rounded down to 109). with a 3 pulse, it's a rate of 300Hz at 6000RPM. period of 3.3333333mS and a timer value of 218.45(which will get rounded down to 218). that makes the error with a 6 pulse .2%. with the 3 pulse the same .2% error.

with 12: 6000RPM = 1200Hz, .833333333mS period, counter value of 54.6(which will likely get rounded down due to timing logic accumulating). so 54. an error of 1.14%
with 24: 6000RPM = 2400Hz, .416666666mS period, counter value of 27.3(round down to 27), error of 1.14%
with 48: 6000RPM = 4800Hz, .208333333mS period, counter value of 13.65(round down to 13), error of 5.03%

GM started adding more notches into the reluctors for the same reason that the single injector TBI 1.0 geo metro gained a cam sensor in 1996: misfire detection. it's a lot easier to track crankshaft acceleration and deceleration when you have multiple reference points to measure during a revolution.