Hav you seen the datasheet?
It clearly mentions that the TIMSK register is common to all 3 timer channels
but the bit numbers have been divided between the 3
like u need to change bit 5,4,3,2 in order to affect the 16 bit timer interrupts....
similarly bit 1,0 affect the TCCR2 timer interrupts
sit 7,6 affect TCCR0 timer interrupts